The Kosmokrator of Beasts has granted to three PC magi dominion over the Animal Form. One was granted dominion over large animals, one over small animals, and one over animals that are neither large nor small. A major magical focus seems about right in scope to cover a third of a Form (33-34%), since by RAW anything less than 50%, and at least 20%, qualifies. As a SG, you have to choose the size below which animals qualify as small, and that above which animals qualify as large, so as to "split" the Animal Form in three parts as fairly as possible for your three players.
This is tricky, in that there are many considerations involved. Larger creatures are rarer; at the same time, a bonus with dragons might be worth more than a bonus with bees (maybe). It's also clear that -- as for all Virtues and Flaws -- the relative value of one focus vs. another is somewhat saga dependent. Just do your best assuming a saga as "average" as you can imagine... or maybe your current saga. Once again, your goal is to choose the "small" and "large" thresholds so as to make the three Virtues as equally valuable as possible.
The above is just a warm-up! In general, this thread is about how to split Forms along a quantitative axis in three "equally valuable" parts for the purpose of a major hermetic focus (or perhaps potent magic). Finding a good axis is part of the problem. Such an axis should:
a) cover the entire Form. Temperature for Ignem does not qualify: Ignem is also e.g. light vs. dark.
b) be meaningful, if potentially a little fuzzy, for game characters. Size for Animal or Beauty for Corpus certainly qualifies. Damage Modifier probably does not.
c) have a clear, quantitative representation in Ars Magica mechanics. Size for Animal or Corpus certainly qualifies. Beauty for Corpus does not, because Presence does not quite capture it.
I'll edit this post listing concisely the proposed axes in the thread, and the proposed splits. You are certainly encouraged to express your view on how to split an axis proposed by someone else. Splitting Forms in this way might not be to anyone's taste; this thread is not about discussing whether you'd allow it in your saga (go here for that) -- it's about coming up with "reasonable" axes as per a) b) c) above, and splitting the Form in three as equitably as possible for the PCs.
Large animals, small animals, animals neither large nor small (ezzelino)
large: size +3 or more; small: size -3 or less (ezzelino)
large: Size+3 or more, possibly less; small, size -3 or less, possibly more (YR7)
Large folk/bodies, small folk, folk neither big nor small (ezzelino):
large: Size +2 or more; Small: size -1 or less (ezzelino)
Old, new, neither new nor old (ezzelino):
old: a score years or older, new: a year or newer (ezzelino)
The greater supernatural, the lesser supernatural, the supernatural-in-between -- basically supernatural "weight": magnitude for mystical effects, might magnitude for Realmed creatures, level for auras, perhaps highest Art/5 or highest supernatural Ability/3 for magicians? (ezzelino)
greater: magnitude 9 or more, lesser: magnitude 3 or less (ezzelino)