What magnitude is a Muto Corpus change in size for a human?
A change to arbitrary size is still Base 3.
Base 3, but the spell must have a sufficient Size modifier to magnitude to affect the final form.
Base 3, +1 magnitude for every +1/-1 size.
Base 3 allows changes up to +1/-2 in size; every additional +1/-2 adds 1 magnitude.
Other (please specify)!
A recent thread made me think of how easy it seems by the RAW to turn people into giants. It's only MuCo 3 to "utterly change a person's size". So technically a magus (or, more scarily, a first year apprentice!) could change himself into a 100' tall giant for D:diam with just a level 4 MuCo spell (ok, with requisites for clothing, but at those levels it's no big deal). So, most troupes I've seen houserule it one way or another to make it a bit harder. The most common houserules I have seen are:
a) The spell requires sufficient magnitudes to affect the target in its final form. So any change to a size of +1 or smaller is Base 3; since every additional magnitude increase the allowable mass by a factor 10, and thus the final Size by +3. Thus, changing to Size +2 to +4 would add 1 magnitude, changing to Size +5 to +7 would add 2, changing to Size +8 to +10 would add 3. I find this a bit too generous, both because turning oneself into a fearsome Size +7 giant is still easier then turning into a wolf, and even more so because turning really really tiny (which is quite effective for stealth) is still only Base 3. Also note that this is not compatible with Preternatural Growth and Shrinking, which can turn a Target of size +1 into one of size +2.
b) The spell simply requires +1 magnitude for every +1/-1 change in Size. I find this a bit too hard, particularly for the smaller sizes. Also, what you can do with Preternatural Growth and Shrinking is definitely better than this.
c) Preternatural Growth and shrinking sets what you can do with the Base 3 guideline: up to +1/-2. Any additional +1/-2 is 1 magnitude. This is what we use, since changing into something the size of the smallest (size -10) or largest (size +5) land animals ends up being about 1 magnitude for a size 0 human then changing into the corresponding animal (though a human form is generally more useful and you need no Animal requisite).
This is RAW, but I must admit it took me a nasty amount of searching to get to this result.
I'd personally have prefered
though with the note that a specific spell does one or the other, not both.
Careful readers will note that Preternatural Growth and Shrinking has a +1 magnitude for being able to do both, which satisfies me.
If I can find it again
It about sums up to "you must include appropriate size modifiers such that both the initial object/individual/whatever and the resulting object/form/individual/whatever would be within the size range allowed." - which I rather dislike, because there's no limit skrinking suff.
I'll try to find the actual rule, but I have a game today (2 actually) so it might not be today.
To anyone else who goes looking for it, happy hunting.
Are you sure you are not referring to the Conjure guidelines in RoP:F? But if that's the case ... that's not really Hermetic magic, so you can't quite apply automatically it to Muto Corpus. Besides, it does have limits to how small you can make stuff (making stuff big requires the extra magnitudes, making it small requires higher level guidelines).
Thanks for the pointer. Once again, Magi of Hermes proves itself the low point of Ars Magica's quality control.
In addition to adding one magnitude because this Muto spell can change the target's equipment (by the core book, you only need casting requisites) the spell also says "A size modifier is required for this spell since the final size is ten times the mass of a base target for Corpus". Since the final size is not ten times the mass of a base target for Corpus, I think this sentence should not be taken into consideration.
Incidentally, +1 magnitude per +1/-2 size (beyond the initial +1/-2) is the guideline that would yield the current final level of the spell as it is
By RAW only worn equipment is transformed not stuff carried in hand. It makes sense for the spell to transform the character's held weapons (the character is an Axe weilding melee type) so I assume that's what the +1 is for. As far as final sentence goes yes the target does end up 10 times larger. +3 to the size characterisic is x10 mass multiplier. Pg 18 ArM5.
So if I cast the seven league stride, my magic wand (unless it's my talisman) stays behind?
If I make my grog invisible, he'll be invisible but his unsheated sword won't?
If that's the case, then I've probably been playing a very houseruled Ars Magica so far.
Well I only double checked the MuCo guidlines. (Sidebar ArM pg 135 sorry I didn't cite it before.) They specifically exclude carried equipment. If I recall the discussion of invisibility in HoH:S swords where not automatically excluded.
Possibly. Or it's a cut and paste error from some other spell.
But regardless of the source of the author's misunderstanding, I would not take a sentence where the author has clearly messed up with the guidelines as an exemplar.
It's a R:Touch, T:group verion of the same spell, by the same magus, in the same page, with the same description - except that the entire offending sentence, the one that explained (with faulty math) that the effect must be able to affect the "final" form, has been removed. I'm not sure how this weakens my point (that it's clear that the author did not pay attention to the guidelines involved, so whatever he wrote should not be considered as a source of insight on the guidelines).
More in general, MoH is one whose spells and effects appear the most full of errors of the entire line. To be lenient, I'll just assume that the magi involved experimented a lot, so the outcome is not coherent with the guidelines. But even in this case, taking one of the spells involved and drawing from it general conclusions about the guidelines is a long, long shot at best. Give me one example from some other product, and I'll be happy