Maybe it was discusted before, but i could not find the topic.
If i am not wrong there is no way to create an "area of effect" as a target for a spell, exept for the boundary and circle.
If i would like to create a spell that burn everythig in a 10m radius it would not be possible...
Or am i wrong ?
You are right, but there are ways to bypass this.
First, "material" Forms can be made large.
So you could create a fire large enough so that it have 10 meter radius. That's about two magnitudes more to do that. Each giving ten times more of stuff, the basic fire size being roughtly about one square meter)
So yes, it is possible to create spell that burn everything in a 10m radius !
Second, you could use particular and strange spell designs. Some will consider this to be house rulings. In any case, there is danger for game balance here.
You can take for exemple this spell: Thread the Ashen Path (p. 138) with the modifier "+1 fancy effect (the spell ... keeps being cast ...)
This spell basically permit you to "create" an area of effect that follow you, within which plant material is destroyed.
This is not a real "area of effect", but still it achieve the same.
FOr 1 ) the size of fire, i have to disagree.
A fire is quite a circle. But not really. I see this like a half sphere.
A "campfire" is a half sphere of 40cm diameter.
For the ease of the math, and the visual enjoyement, i decide to go for a sphere of 20cm x2 diameter.
So, with 2 magnitudes, you * the mass by 100.
In diameter, it goes from 40 to something like 1m80 (don't remember my calculations, sorry).
So you will need a big mathematics process to find your "10m diameter fire"... and it would need at least 5 magnitudes i think.
id prefer go for a multiple fires with some magnitudes for nomber. Like 1000 "40cm diam" fireballs (group +2 +2 for number x 100) flying over a group up to 1000 individuals ^^
So basically there is no target "area" in Ars Magica.
No let's say that my magi want this effect, having the ability to target "somewhere" instead of "something".
It is covered by the breakthrough rules as a minor breakthrough.
Let's say it's equivalent to "group" and target a sphere of 2 paces.
do you think it would break the game in any way ? It would be some kind of Hermetic virtue (minor/major?).
I dont understand what you want.
If you want to cast a fireball in the sky, you can.
If you want to launch a big fireball (with said, +5 magnitudes for HUGE firenuclear ball), the most far , you can.
If you want to cast 100000 big pilums of fire on a forest, you can...
You need:
And that covers every possibility, i think...
If you want to cast a fireball in the sky, you can.
That's true, and if i color them i can be Gandalf 
If you want to launch a big fireball (with said, +5 magnitudes for HUGE firenuclear ball), the most far , you can.
This is true for Ignem because the basis is a camp fire and by doing a BIg camp fire it works. But if i want a water ball or a wind ball..........
If you want to cast 100000 big pilums of fire on a forest, you can...
Yes but it's quite different 100000 targets or 100000 peoples on the area of the spell.
Take the guidelines...
for a "waterball", its would be a Cr(Re)Aq because with creo you create the water, but "naturally" water can't have the form of a ball, IMO.
Wind ball. It's quite strange, but i would say same Cr(Re)Au.
I really don't see the question/issue here, sorry for not helping you...
No no problem it may be me not being able to express myself corectly
Rather than trying to be very general,
list a few specific effects that you want for area of effect (size, effect) and we can see about creating the spell for you so you can see some actual examples of how to do the area of effect.
Mostly correct.
Circle and Boundary are "areas", ill-defined as they may be, and there are some more exotic targets that have been explicitely bounded in size (e.g. the Road and Road Network Target of the Neo-mercurians that affect everything on a road or road network ... within certain bounds).
But indeed, there is no 15' by 15' square area target. However a Formulaic ([i]not[/i) spontaneous) can use non-standard parameters. It is generally considered "somewhat more difficult than the equivalent target". So if you want your spell to affect an area that is about the size of a standard Room, your spell will end up be somewhat higher level than one that uses a standard Room target. How much harder is left to the appreciation of the Storyguide, and need not actually be harder, nor be a whole number of magnitudes (though "+1 magnitude" is more or less the default choice).
Remember that this is not possible for spontaneous magic which must use standard Hermetic guidelines - or guidelines otherwise explicitly made available to the magus, from things like Faerie Magic, initiations or breakthroughs. If you want your magus to be able to use "15' by 15' square area" in spontaneous spells, he must perform Original Research and achieve a Minor Breakthrough. Then he, anyone apprenticed under him or anyone learning a formulaic spell he invented using that new target will be able to use it effectively as a new standard target.
Without a well-defined boundary (Circle, Room, Structure, Boundary) you can't arbitrarily place your spell and affect a bunch of individuals in that area (e.g. mind-control all humans within 25 feet of that tree). Unless its with something like a single, large fireball, or digging a pit of a given size, or turning part of a lake into ice, in which case you rely on the standard individual size for each form and tack on magnitudes if you need it to be larger.
In most cases, and unlike in That Other Game, you don't usually try to place your spell "just so" to affect a maximum number of individual targets - when you do, it's because you're really affecting a single, smaller or larger, individual and using it to do stuff to the hapless victims in the area (and make a Finesse roll to see if you place that individual correctly). If you want to mind-control a bunch of humans, it's either a Group, an explictly-bounded spell (Circle, Room...), an exotic Target (Road Network, Body of Water...), or a special target in a formulaic spell.
It probably won't break anything, there are way more outrageous targets, but you still need to decide what it means exactly.
If I cast a CrTe to create, say, diamond. Does that mean it creates a single diamond of that size? That it fills it with individual-sized diamonds? Keep in mind that the standard individual for precious gems is much, much smaller than it is for loose dirt.
If I cast a MuAn spell with that target to turn animals green, does it affect every individual animal in the area? What about animals that don't fit entirely?
That's the hard part about specifying a new R/D/T: decide what it means. 
Don't forget that Group itself can still affect a single individual. It does cost one more magnitude than the equivalent size-boosted Individual target, but is much more flexible. And you can typically fit more than 10 Individual mass-equivalent of most forms (exotic solid and liquids being notable exceptions) in a 2-pace diameter sphere.
It may get icky when dealing with spirits: how many angels can dance on the head of a pin?
Minor at best, or possibly available from the right sources.
Want to try me in French?
It may be my fourth edition roots showing, but I was under the assumption that you most certainly can use these alternate parameters in your spontaneous spells if you have the appropriate Virtue (such as Faerie Magic). I may be wrong. At work, not near books, I invoke Smurf's Parma.
Which is precisely what I said: standard hermetic guidelines or guidelines explicitly made available to you.
Ah! Okay, I reread it. Syntax error.
I am busy fighting off Frost Giants and the Snow Queen 
My shadow froze to the ground this morning.
I though the target for creo was the thing created ?
I could not use an "area" target in a creo spell. I don't think i could use a "road" target for a creo spell it would be a muto spell and i think it xould be very hard to transmute a whole road into diamond
Yes it is, but, well... Sometimes, it gets complicated 
(And you're french, too? Welcome! 
)
Yes. Creo spells work differently than other spells. Read the "Targets & Creo" guidelines, sidebar page 113.
Combined with the basic "amount" from any Form Guideline, you can "create" X amount and place it anywhere. Because that "amount" is defined, it can take up an area.
For Muto, Perdo and Rego, you want to use the Target:Part or :Group and a size multiplier.
I've rarely had a problem achieving such via the Rules as Written (unless the "problem" is a magnitude that is too high).
If you are having trouble with a spell effect, maybe you can give an example?
I am french and our covenant is on the shores of the Lot river 
nods knowingly
It's not a problem of magnitude, it's just that i am so used to magic systems with area of effects tha i was curious when i saw that there weren't any in Ars Magica or at least not in the usual fashion.
If you think about it, most magic systems with clearly defined areas of effect are... how to say it... more rooted in tactical combat (or are successors to games which are).
When you have a hex or square battlemap and you're movement around the board is done a few squares at a time, determinig that a spell covers the nine squares the enemy occupies is pretty important. These rules are also a limiting factor on the power of spellcasters vs other character types.
Ars Magica was a direct rebellion against a certain well known game like that. I think part of that rebellion was having less emphasis on tactical combat and much less strictly defined parameters for spells. Additionally, Ars Magica deliberately tossed the concept of balancing spellcasters with other characters right out the window and tried to get it to bounce twice when it hit the ground.
A Creo Ignem spell with Individual target can still create a ball of fire big enough to encompass Norwich Cathedral if the magnitude is high enough.