OK folks, this just leapt to mind
If I use mentem to make someone calm down, I do not, ever, have to add a size req. A single mind is the default size for mentem, so a persons mind is size 0 regardless of the size of his body.
But how about for animal? Does a ReAn spell to calm an animal need a size req to work on bigger beasts, even if it affect their minds? Are animals even considered to have minds or are they considered to be purely physical?
Essentially, I can make a size +1 beast calm using animal without a size req, but would I need the size req to affect a size +2 animal as I would if i was using magical to physically pick it up?
While animal minds are considered in ArM5 spells (e. g. ArM5 p.120 box), there is apparently no size related exception for Animal, like it is for Mentem (ArM5 p.148).
In particular there is on ArM5 p.120 ReAn 25 The Gentle Beast, calming animals up to Size +4 with a +1 size modifier in the design.
I will not check for consistent application of this throughout the books, though.
EDIT: this certainly has nothing to do with A&A's summary (p.31f) of medieval ideas about the mind, which covers concepts about the animal mind as well.
I do think that this spell was by mistake added the +1 modifier. It isn't logical with the mentem part.
Whether it is 'logical' - considering scholastic ideas of the mind - can very well be disputed. But the ArM5 designers clearly reserved the (ArM5 p.148) exception about size modifiers to Mentem. Otherwise they would have written a similar exception into the introduction to Animal spells (on p.117), and removed the size provision on p.120 The Gentle Beast.
If you do not like it, talk with your troupe about a house rule: I don't see how it would break the game.
EDIT - to spell it out clearly: From a scholastic perspective it is 'logical' to do away with the Animal Form in Hermetic magic completely, put the body of animals under Corpus and the animal mind under Mentem.