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).
Possible, but I'd be surprised.
I was at the time looking to be particularly pedantic about size changes via Muto :-/
Magi of Hermes has a spell that seems to work this way on pg 49. It's a range personnel effect invented by a size +0 magus yet it has a +1 size modifier.
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.
Base Individual for Corpus is size +1, not size 0.
So 10 times that is size +4.
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.
And that justifies ignoring the whole sentence. Certainly your not suggesting that the size modifier to the spell shouldn't be needed because the final result is quite as big as it could be.
It makes it clear that the spell is written without paying attention to the guidelines.
So, I would not infer anything from that sentence about how to apply the guidelines in other cases.
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.
Or it's a minor math error.
Ah, interesting. So if I have a sword in my hand, it does not changes with me, but if it's in the scabbard on my hip, it does - and so does a gauntlet. Weird.
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.
So we'll just ignore that sentence and look for another exemplar where they get the math right. Hey look. Turb of the Giants on MoH pg 49-50.
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 
Incidentally, I'd point out that assuming that the spell must be able to affect the final form of the Target is incompatible with Preternatural Growth and Shrinking, which can turn a size +1 human into a size +2 human!
Yees, that bothered me as well.
And no, I still haven't located where I saw it the last time.
Having seen a grugach in play using this kind of effect heavily on grogs in a recent minisaga we have seen how powerful it can be. Gruagachan have the additional effect that equipment and the like do not imply extra magnitudes/requisites, so they are even better at this, but the general premise is that size modifiers are needed for larger sizes. Base 3 is useful for a size increase up to +1. You need a +1 magnitude for size +2, +2 magnitudes for sixe +3... So the scale ends up being like this:
B3. Increase size up to +1
B4: increase size up to +2
B5: increase size up to +3
B10: increase size up to +4
B15: increase size up to +5
B20: increase size up to +6
Add range, duration and requisites for equipment on top of that.
Ogre sized turbs or above are quite something if you just want killer power, even if they are not easy to cast: Touch, Sun individual is MuCo30 adding Metal and Herbam requisites to change a grog to size +3, so going for larger grog groups tends to cause warping.
Cheers,
Xavi