On a slightly related note. p. 192 says that an increase in size of +3, is equivalant to changing it by a factor of 10. Assuming thats refering to bulk, (rather than length) then a +3 increase in size would only increase a spells magnitude by 1. So, a change in size of +9, would only add 15 levels to a spell. That's almost a bargain 
/me mutters about his tyranical GM 
Next question, opinion seemed divided on adding ignem and auram pre-reqs to give the dragon firebreathing and flight.
Prystus's objection is that you can't use an ingnem pre-req to add fire breathing, as that's an entirely seperate magical effect, and a pre-req can't add a seperate spell..
my submission is that fire breathing (and flight) aren't spells, they're natural abilities of the beast I change into... it does have wings capable of supporting it in flight, and it does have a fire sac next to it's lungs containing the right mix of natural chemicals to produce firebreath.
So both functions are the animals natural abilities, rather than seperate spell effects. Just as I wouldn't need a seperate magical spell to make my falcon fly or my shark breathe under water.
(and yes I know they're not 'natural' abilities as such, but that's what I'm paying the extra 2 magnitudes to make this a mythic beastie for).
So, when I then add the extra magnitude each for the ignem and auram.. does that cover the firebreathing and flight?
I agree, they are abilities of the "highly unnatural" creature you are turning into.
While it may seem cheep to make a spell that appears to do multiple things, just by adding requsites, remeber that your casting total uses the lowest art required (unless you have a virtue saying otherwise). So a muto animal level 50 spell with a requisite of ignem and aurum, would likely require all of forms to be high. (i'd be impressed if you had a high enough technique that your forms could be low)
Perhaps TotRBttTT is transformin an Animal Individual of up to standard size (up to +1) into another 'standard-sized or less' individual Animal. What you're seeking is a spell that turns a +2 sized target into a +12 sized target, so you pay for the higher of the two (adding magnitudes to reach size +12).
At +1 magnitudes per +3 size increase, you will need +3 magnitues to increase your target's size to +11.
I am not entirely certain if transforming into a dragon is an appropraite spell effect or not. You can turn a horse into a horse with wings, but can you turn it into a pegasus of Might? I don't see the guidelines allowing you to do so, and I do think Hermetic magic cannot confer general magic resistance (such as Magic Might) except by Parma. So I would rule you could add qualities making you more like a dragon, like wings, but never become a true dragon, a creature with Might, without some Mystery or Breakthrough.
I would peg the spell as a powerful MuAn effect (more than the base 25 needed to sprout wings to a horse); with +2 for D:Sun and +3 for Size +11, we reach spell level higher than 50 - a Ritual.
I don't think adding the ability to fly requires requisites. MuAn already says it can add wings to a horse, I'd say if requisites were needed to make these useful the guidelines would have noted that. With the wings comes the ability to fly.
I am not sure about adding further qualities, like fire-breathing. I think I would err on the side of caution and rule that this is a Power of dragons and thus cannot be changed into; just invent a Pilum of Fire and cast it without words (I might be argued to allow the breathing to serve as gestures, for this unique spell, just for the fun of it, at the cost of disallowing gestures in non-dragon form).
"my submission is that fire breathing (and flight) aren't spells, they're natural abilities of the beast I change into... it does have wings capable of supporting it in flight, and it does have a fire sac next to it's lungs containing the right mix of natural chemicals to produce firebreath."
Muto, and thus mutation of an existing form. doesn't give that existing form the abilities of the form you change into. That's kinda the province of the Heartbeast ability of the Bjornear.
The only time this is validated through the rules is the creation of a creature in it's entirety from mothing through Animal effects.
You could create a fire breathing flying, earth tunneling mystical(Maybe, nto sure about the mystical part) beast with animal[the effsct description indicate this], but you could not Muto the same base animal into said creature with a single spell.
It's the functional differnce between creating the thing from nothing and giving it things, and making fundamental(If temporary) changes in something that already exists. The two really just are not the same thing.
And the rules back this up in the way the effects are listed. Honestly. a Dragon has a LOT more than a +1 or +2 stregnth. I wouldn't expect a spell though that changes you into a Dragon to change your strength score as well. Once again. that's the province of the Heartbeast form. NOT a spell effect installed into another one. It would require an entirely seperate effect added into the series of casting to get it. Which would be Creo Animal.
Basically you are not functionally creating the building blocks for a thing and bringing it into existence with Creo Animal. You are trying to add modular special effects to a thing that already exists. No matter how you slice it, the two are not the same. Too many effects into one spell.
Gimme an example of an existing spell, basically anywhere, that does three or four entirely different things[Crossing multiple Techs and Forms] to the SAME target, in ONE casting, that already exists[which is exactly what you are asking this spell to do], and we'll talk about the realm of possibility within the context of the rules as they sit.
THis caught my eye as well.. now that I think of it "natural abilities of the beast I change into"
Yes I woudl agree as well, of the beast, not fo the Magi. THis backs it up even further. The HEartbeast a very sopeical power allows for the assumption of the natural abilities of the animal in question, the creation of such an animal also allows giving it the powers in question, but as a base starting thing, you as a Magi are not a Dragon. And unless you have a realy strange Storyguide, you are likely as not, not a Bjornear with a Dragon Heartbeast. So any attempt by yourself to become a dragon is a matter of changing and adding onto yourself through magic.
Thus in all honesty. You need to -
Change your form,
change your size(fortunately both are Corpus),
make sure you have the hide and armor of a dragon(animal once changed),
give yourself flight(Aurum),
give yourtself SPEECH(Dragons can talk, at least i've always assumed they can),
Give yourself some kind of smokey scarye breath(Uh... Aurum something or other)
Give yourself flaming breath((Ignem),
and probably a load of other things I could think of given just a little more effort[like night vision for instance]. None of which can all be lumped into a single category of spell. Okay. all the physical changes I might buy in the Animal area with Prereqs for.. uh. Creo, Muto, and possibly Terram(Armored skin).
It's just far far too broad for a single effect. Even a ritual one. Hell one of the MOST pwerful rituals{Non_Mercurian] I can think of only ports people from stone circle to stone circle[Hermes Portal]. Or if we delve into Mercurian and Quaesitor, what? Gives bad luck[Screw the Diedne anyone?}? Okay... one kills a family/Lineage line. Still even then you are talking only one or two techs forms, and still only ONE given effect.
Okay, I'll stop rambling.
When I turn into a falcon, I can fly. Not because I cast spells, but because the form I turn into has muto'd wings of the right dimensions, and the right muscle structure to power those wings. It's a physical thing, not a magical thing.
Why would that be different with a dragon? If it has the necessary wings and muscle structure, why can't it use them? In fact why would it need a pre-req to use them.
I don't think it is different. (See my example of a horse with wings, which by cannon does not require a requisite.) But this is not the same as adding the ability to breath fire - breathing fire is a Power for a dragon, it is a supernatural not a natural thing.
Dragons are not animals, their fire is not due to checmistry (which doesn't exist) but rather to magic. It's completely arbitrary, yes. But Mythic Europe just doesn't follow our kind of nature, it follows a Mythic reality where dragon breath is a power with penetration and aligned with the Magic realm. Wings on the other hand are just natural qualities, found in mundane animals, and it is such natural qualities Muto plays around with.
Note also that it has little to do with muscle structure, wing geometry, and aerodynamics (which says dragons CAN'T fly). Even though the wings for a horse are realistically too small to let it fly, it WILL fly, since it's mythically appropriate for a creature with wings to be able to fly.
I can live with that..
flight comes with the wings (and without the need for a pre-req), but I can't get fire breathing even with a pre-req.
For that I'll get a seperate spell, or an item made by the elementalist I hang round with.
as a dragon is a mythgical, and magivcal creauter. I rather imagine the power of flight is alos a magical power as well. Not a natural one.
Of course we are speaking of a creautre that is in and of itself, entirely mythical to being with. Which in and of itself, breaks the rules of animal anyway. As animal stands. you can't change into a mytical beast[unless you are a Bjornear moving along an Inner Mystrery]. That more than anything would tell me that taking the form would be a peicemeal spellcast issue at best.
The upshot is, you'll have to assemble the various parts and powers of a Dragon, and apply them to yourself. And you just can't do that in one, perhaps even two spell castings.
BTW, in a hawk's form, you don't need magic to fly. And a Hawk, isn't magical or mythical by it very existence. That is a major differnce of issue betwen it, and say. a chimera, or basalisk. or dragon. Given an individual storyguide. I might. might find some reason to give off flight with a dragons shape, but it's not likely. AS it's not a natural entity to start with. There's no natural "base form" to work from. It's all magical.
You're giving way too much credibility to the arts of Ars and thier ability to handle a given situation in a single spell.
Given a major breakthrough in Animal I might allow the assumption of mythical forms[since Animal doesn't allow for that], but to get any ability assocaiated with that form, would require more spellwork.
Had my say though.
Rightio time to get stuck into this again.
I would rule (and since its my saga, haha, i just have) that you can add an ignem requisite to get firebreathing, if (IF!!) the beastie you are turning into can breath fire. I base this on a sentence in the Creo Animal guidelines
I don't see that there would be a real difference in the rules between creating an animal and turning something into the same animal.
Some minimum familiarity with what you're trying to turn into should also be required.
Can't say that I can find a Hermetic Limit as such that would disallow turning into a dragon temporarily. Granting magic resistance as such is perhaps not possible, but creating a mythical beast whole cloth with magic might is possible with Animal+Vim.
I'd rule turning into a (real, whole-cloth) dragon with no magic might, but with powers activated by spending fatigue, quite possible. But since it is a magical beast, I'd use the "create a magical beast" lvl 50 CrAn guideline as an approximate difficulty of the feat.
A simple spell for turning into the example dragon from the main rulebook, sans Might:
Stellatus' Brother Come Visiting
R:Per, D:Sun, T:Ind
MuCo(An,Ig,Me,Vi) 95 Ritual
(base 50, +7 size, +2 Sun)
Vim requisite for the "magical beast" part, Mentem and Ignem for its listed powers. No additional magnitudes for the requisites, since they simply allow the spell to do its job.
And no, for the example dragon, flight seems to be natural - since it isn't listed as a power. So no Auram requisite, just as there is no Auram requisite for turning into a bird these days. (All right, I only assume it can fly based on the illustration.)
(The Limit of the Soul suggests that this might cause the caster to switch places with the real dragon, though, instead. An obscure reading, but a theoretical danger for the magi to debate.)
Now, you Bonisagus lab rats: how long for someone to actually research this spell? I think a lvl 95 six-art spell is pushing at the limits of possibility. 
I'll do what I see as a likely case scenario. A magus in his seventies who has an applicable magical focus, if the magus isn't focussed towards muto, he's not going to squander his time failing to get the job done.
We'll give him pussient muto, affinity muto, and a magical focus in self transformation. (Actually this is very much like a character that I have in my present game)
We'll give him a muto of 40. I think this is justified, 50 years out of apprenticeship studying one season per year for an average of 15 xp per season study would give us 750 xp, add in some more experience from exposure, adventure, and apprenticship and you've got plenty, it would "only" take him 703 xp. to get to level 37 (40 with puissant).
we'll peg the worst of his requisits at 15.
magic theory 9 specilaized in spells (consevative by my reckonning)
Int 5 (boosted with creo mentem spells)
a lab with a muto specialization of 4 and a general quality of +2
Aura +5
so right off the bat this gives us 96 (enough to finish the spell in 95 seasons)
Let's give him a similar spell bonus of 10 (he''s learned at least one self transformation spell right on the edge of ritual required territory)
we'll give him some help in the lab
a familiar with int + magic theory of 10
an apprentice with an intellegence + magic theory of 7
An old friend who owes him a favor and wants to get a copy of the lab notes with an intellegence + magic theory of 13
the friend's familiar with int + MT of 9
96 + 49 = 145 They'll complete the project in two seasons.
You could add in hermetic astrology, or somthing like inventive genious or puisant magic theory for either of the magi or the apprentice.
You could have cyclic magic add to the lab total, you could further refine the lab, there are lots of ways to get this total higher.
You wouldn't be able to do this with a character that wasn't a muto master but the character's magical focus only adds fifteen to the total. You could find a way to compensate for not having an applicable magical focus.
What makes this possible is that all of the requisites are forms. If there were a technique requisite you wouldn't be able to generate as high of a total. (Of course a character with a sufficently large leadership score and enough political power could get the job done with a huge team of researchers)
Intresting aspect: using powers requires might. No maga will have any might, and so will not be able to use powers - thus no firebreath...
Physical stats will change however.
(Oh, and remember that in your new form, you are still affected by corpus spells, not animal - because that is your true nature. Only heartbeast avoids that one).
Corpus spells and animal spells, not just corpus. (from Mystery Cults)
Given all the wrangling over hermetic/non-hermetic on this topic, here's a solution...
Make it a Mystery, with the final, Achieve Immortality mystery of the cult being the ritual to actually become a dragon (a momentary Muto ritual - Mysteries can violate hermetic limits, after all). That way, it can be as non-hermetic as you want it, and still do the job. Still, you'll pay a hefty price for your knowledge (did I mention you'd have to self initiate yourself in your cult's mysteries...? Ouch...).
As an aside, I would say that changing someone into a Dragon, along with a Vim requisite, means they have the might to use whatever powers the new form has - that's what the Vim requisite is for. For a magus, I think this would be unnecessary, because they would still have thier gift, and the Parma Magica.
Regardless, this would probably be better if it were put into an item.
Steve
An interesting idea for a cult. I'd make the effect a ReMe(PeCo) Ritual, with a preceding CrAn(Vi,Ig) spell to create the new body. I'd require other magi murder you and cast the ReMe ritual, making it far harder for the cultist to reach dragonhood (in line with The Elixir precedent). But yes, a very intriguing path to immortality there.