And we reach the famour Hermetic Limit of Being Eaten by Dragons For Doing Stupid Things That Break The Setting and we are off to continue the saga 
I'd originally looked at 183m as 183 miles, silly US centricism... 
Anyway, upon further reflection +6 size is still too low for this spell. +6 size should be 1,000,000,000 cubic paces (100x100x100). We'll call 3 feet=1 pace=1 meter, for simplicity, technically a meter is a bit larger than a pace, but at this scale, it's not really worth it, and actually only strengthens my argument that the size is too low.
Formula for volume of a sphere is 4/3πr^3, or 4/33.1491.5^3=3,207,241.53 cubic paces. That's in excess of +6 size, pushing it to +7, which pushes the spell to a ritual. Or not, if you're dropping the Rego requisite.
There's still not a lot to like about this spell. Dropping something that is described as being the length of a soccer or US footbal field in length, width and height onto someone is going to cause some problems. It's likely to squash a lot of mundanes, wildlife, forests, parts of villages and make life more miserable for the casting magus. And without the Rego requisite, you need that finesse roll, and better not have anyone you like without adequate MR within range of the volume of effect.
Sure, it's within reach, but there are often better spells that benefit the covenant. Especially one like this. I typically don't like to invent spells unless I can do it in two seasons OR there is a powerful need for the spell. Again, this kind of spell is going to only interest someone who is interested in creating odd effects, a terram or creo specialist or someone who really needs it. There aren't going to be a lot of characters who fit in those molds...
That same person could benefit from creating variants of Conjuring the Mystic Tower...
If the purpose is to squash a lot of stuff with a big rock, a disk (or not-very-tall cylinder) is much more efficient than a sphere: it has more squash area for its volume, plus all the squash area will touch the ground, which the sphere's curvature will prevent.
Without working out numbers, let's just notice that this makes it easier, not harder, to create a spell to squash lots of stuff.
All this thought about squashing makes me want to watch Bambi Meets Godzilla again 
I'm with Jabir on this one. Besides, if you use the rules for falling (and impact) it would still do close to +6600 damage, so it's pretty much a moot point anyways.
Definitely, it's not something you'd create just to have up your sleeve. A saga where you'd actually have a use for a spell like this doesn't come around often. Doesn't hurt to be prepared for one though 
I've always used 1 pace = 1.48 meters or 1.5 when i want to simplify it, which i didn't want to do in this case considering the huge numbers. So 1 cubic pace = 3.2 cubic meters. Seems about right.
Using a sphere just makes it a lot easier to calculate since you can mimic it as an asteroid impact. The crater gives the spell an extra oomph and i'm not sure how to calculate that for most other shapes. If you make a disk and assume you can assure it won't turn mid air and want to affect an area with the minimum size of the crater, its dimensions would be 400m in diameter and 25m in height. Height is important for impact depth and this disk would only penetrate like 10-30m in total which would most likely add very little to the width of the final crater. While still highly effective it would leave a very different crater.
There's been discussion on this before. A pace is roughly 3 feet.
There are (iirc) 3-4 citations in the core rules which indicate a pace = 3 feet, exactly. I know because I too, until recently, operated from older beliefs that it was vaguely defined. It's not formally "defined" in so many words, just clearly applied as such repeatedly.
Cool exercise!
(I woulda bet vis you were over-estimating the "yield", but I got 46 ktons, using a drag coefficient of .1 (smooth sphere). And I woulda lost!) 
That decision would have to be relative to the other effects in your Saga, but if ims then it certainly would be a Ritual, yes. (I'm not sure where I'd draw the line ims, but anything even near this powerful would be, even if well below level 50. I"m not comfortable with anything of this power-range being a Formulaic spell.)
To some extand, it will.
Apply +35 damage to any normal character or animal, and he's dead. So it doesn't matter whether he took +35 or +35000.
I don't remember the rules about material durability, but it should also at least seriously damage any building.
Note, also, that, surprisingly, it's not THAT huge.
100 paces in every direction = about 30 meters.
From a modern understanding, such a stone falling from the sky would obliterate a city, sure, and then some. Blast zone and all...
From a medieval one? It'll probably crush the thing on which it lands (a castle, a city block), and that's about it, simply because there's no E=MC² there. That's the big, big, big difference, and what's making it difficult for us to think otherwise.
If it can help, think fantasy and movie: Unless in the rare realistic setting, if such a rock landed on a dragon, it would hurt him severely, not kill him. Which is possible with +35, not with, say, +50000. So you drop your giant stone on the Black Dragon, there's this great fracas, and, when the smoke dissipates, here it is, bleeding, with broken bones and tattered wings, but still alive (and either quite angry, or afraid)
I thought the total damage you took from a fall was limited. My bad.
OTOH, on the "spells of mass destruction" thread, someone suggested making the spells with more than +3 or 4 size modifiers rituals, which, as this proves again, is probably overall a very good idea.
+35 damage might not kill a knight, even if it will severy harm him for sure
+35 damage will not even make the stones in a castle rattle.
+6600 will :mrgreen:
The "problem" with Ars damage is that it is human-based. If you go to the extremes suddenly you are saying that an armoured guy might have the same resistance than a stone wall has, and this feels quite wrong to me.
Still, I would apply the famous SG fiat here: a rock of this size landing on anything will destroy it and leave a big hole in the ground. Damage rolls not needed. Magic resistance might make this rule not apply or might even dispel the effect (miracle intervention, the dragon negating your power...)
Xavi
PD: In ArM5 1 pace = 3 feet = 1 metre, aproximately 
So 1 pace = 1 metre. Makes for easy ,maths for us, continental Europeans; I always have to think twice what the sizes in feet are, but not the sizes in paces.
As I apply medieval physics , you would get a lot less effect than you would expect as most of the secondary effects you are expecting would not happen, whatever it landed on would be pretty much destroyed and thats about it. If you want a 'nuclear' fireball you need to use a similar Ignem spell, if you want an earthquake then you need an Earthquake spell and any Tsunami from an ocean strike would need an Aquam spell (You would get a wave from an Ocean strike but much less than you would expect based on modern physics). I would also make this or similarly destructive effects rituals and if used against a large urban area likely to fail die to divine intervention , God probably objects to the destruction of all those churches but it would depend on the situation
Because of soak...because of armor? In this instance, I'd say that the armor doesn't apply.
+35 damage is 35 + stress die.The minimum amount of damage is 37, unless the damage roll botches (pg 181 MRB). To be able to survive 37 points of damage, you have to be size +5
Very handy to keep around a few of those dragons... :mrgreen:
Mmm, 100100100=1.000.000? To get 1.000.000.000 you need 100010001000 instead.
Oh yes, definitely worth simplifying like that!
What DO you watch...
Say WHAT??? A pace is NOT equal to feet. ~3 feet is a pace which is about 1 yard or meter.
I meant 1,000,000. I should've just done 10^6. Context of what followed should've suggested I meant 1,000,000 when I said 1,000,000,000 is smaller than 3,207,241.53...
Having actually done the numbers, I see the original spell erred on the paces. I get a mere 62m diameters by a quick calculation that assumes 1 pace = 1 meter. However, it's still too big!
I usually try to leave the physics out of Ars Magica, either Aristotelian or Newtonian. Deliberately creating the rock in the sky to get momentum feels a little too modern to me but assuming there's not momentum at all doesn't feel right either. I imagine if you understood Aristotelian physics well enough you could find ways to abuse it even more easily than modern physics, given that the system as a whole is less coherent.
Ah, i did not know that about a pace. Things seems a lot smaller all of the sudden...
New stats are more like, 124 in diameter (you forgot it's the radius you get Jabir ), 3*10^9kg, 14kton "yield", crater something like 80x320, 3.4 richterscale and an air blast starting at around 800m/s at impact and not affecting a whole lot after 1.5km. Still pretty potent. Or raise the altitude to 6.5km for the original effect.
What if you create the rock just below the lunar sphere? :mrgreen:
I wouldn't want to leave you feeling that way, so try this spell on for size:
Mudslide CrTe 50
Base 1, +3 Sight, +2 Sun, +8 Size
Creates 10^8 volumes of mud, or 10^9 cubic paces of it. Suggested form is a sheet 10000 x 10000 x 10 paces high that drops from as high up as the caster can see. It stays around all day, oozing into cracks and crushing everything beneath.
I leave the kinetic calculations to the more scientific minded and the calculations of larger ritual effects to the more deranged.
Since you've read Injuries, you should know that the minimum non-botch non-crit damage is (35 + 0 - 9 = 26) even if you have no stamina or other Soak source, therefore a Size+2 might survive the Incapacitating wound.
And it is theoretically possible to soak all the damage if you roll a couple of ones. Yes I should've been clearer that I discounted the soak completely. Be that as it may, 90 percent of characters will be a smear on the bottom of that rock.
THe thing is that given its size and weight, 100% of the characters (barring supernatural interference) should go straight into heaven, hell or purgatory when the thing lands Castles and other things like that should be turned to rubble as well. +35 damage does not sound like something that will turn a castle to rubble
Cheers,