This question came up in regards to a shapeshifting magus.
Assume the magus has cast Gift of the Bear's Fortitude on himself, and then shapeshifts into a bear (Size +2) using a MuCo(An) spell, does he still benefit from the soak bonus or does it no longer apply since his size has changed beyond the base target size for MuCo spells (Size +1)?
This extends to the design of the MuCo(An) spell itself: Does a spell to shapeshift into a bear require an extra size magnitude to account for the size of the intended form (as it would no longer apply after your size increases)?
Following through, this would mean that a spell like Preternatural Growth and Shrinking (MuCo 15) would not work on an individual who is Size +1 to begin with, as it would increase its size past the base size limit?
Or is the target size only relevant for the initial target? Would Gift of the Bear's Fortitude work on someone already shapeshifted into a bear via a MuCo(An) spell because their base nature (or the relevant target for Corpus) is a human of Size +0?
The base individual for Corpus is up to +1, so the spell covers a Size 0 or less guy increasing in size by 1 to a maximum of +1. If you are already 1 it can't make you larger, but it can shrink you. A single extra magnitude for Size lets it give anyone up to Size 3 a 1 point increase in Size because it can now handle up to Size 4
To reiterate on this question, given the following scenario, what would happen in your games?
A magus (size 0) casts Gift of the Bear's Fortitude on himself. He subsequently shapeshifts into a bear (size +2).
A. He benefits from the bonus in bear form as well.
B. The spell ends upon shapeshifting into a bear.
C. The spell is suspended while shapeshifted into a bear, but remains active and resumes its effect once he changes back into human form.
He benefits. Page 92 of the core book, in the Bjornaer section it says how Spells already in effect before transformation remain in effect. I take this as a general statement on the rules. You only need to be a legal target when a spell is cast and then the effects remain.
I read that section before and interpreted it as mainly being concerned with the Form of the effect, considering Bjornaer aren't subject to Corpus/Mentem anymore after their transformation. It does not consider Size as a factor at all.
Though there is certainly an argument to be made that spells continue to function even if their original target doesn't apply anymore (like said Bjornaer paragraph or Groups that split up).