Similar to my old question about what animals a Patronus can be, are there any limits to the animal forms an Animagus can be?
We know that the animal form can be much smaller than human (cat, rat, beetle) so I don't think size is a factor.
I'm not aware of any Animagus who transforms into a magical creature (e.g. unicorn or phoenix) so maybe those aren't possible.
Ultimately, what I'd like to know is whether there could be a T-Rex Animagus... (prompted by a funny comment about difficulty with cut/paste while in animal form)