Avatars can be up to 100x100.

However, they are also limited in file size...animated avatars, in particular, tend to often exceed the size limit of 25KB. Most all static (non-animinated) avatars even at the 100x100 size should upload fine.

Larger avatars may be permitted in the future, but they tend to slow down message loading; if too large, avatars may also become a distraction.

Ron