It honestly depends on the theme and how big the avatar can be before it just makes the theme break. 😛
Right now I have 150x150, and I am still experimenting with a longer avatar as far as post-bits go. Luckily myBB has it to where they can upload big avatars but it will resize in post-bits and member pages, so they can actually upload 300x300 and then the software just resizes it to the given dimensions I have it set for post-bits.
On FP, the maximum avatar size is 150x150 and I think avatars looks great on posts. You don't want it too small as people won't be able to see it but you also don't want it to be too big. I think this size is just right.