The typical forum out there is still capping avatar sizes at 80x80 or other similar numbers smaller than 100x100. Going through other forums I am registered at and have frequented in the past, here are their current avatar caps (leaving forums unnamed):
85x85 (Urban photos and lifestyle forum)
No Cap (because all images are auto-resized to a 100x100 box; SimCity fansite)
120x120 (other major Pokemon forum)
No Custom Avatar until certain requirements are met (images are typically 100x100) (other major Pokemon forum)
2 MB filesize limit (all images are auto-resized to a 80x80 box) (major webcomic site's forum)
Preferred 200x200 minimum (since all images are auto-resized to a 96x96 box) (major Pokemon forum)
80x80 (on this one, avatars are a VIP feature only, since the forums aren't the focus of the game) (online browser game)
1 MB filesize limit (all images are resized to scale to fit to a 100x100 box) (legal streaming site's forums)
100x125 (major anime cataloging site)
No Custom Avatar ever (all images resized to 100x100) (major Pokemon TCG forum)
200x200 preferred minimum (all images are auto-resized to 192x192 boxes, though this forum changed their forum software very recently) (other major Pokemon forum)
This is just of the forums that I remember exist and can remember my logins for. Bolded are Pokemon forums for easy comparison to other fansites.
As you can see, PC's regular member limit is already on the higher end, especially with regards to what will actually be displayed in the postbit. The only one higher for what is displayed in the postbit is that last Pokemon forum. Granted, this is a small sample size of forums, but 80x80 to 100x100 is typically the norm for avatar limits on forums (other forums I've frequented in the past are just escaping me at the moment, but the vast majority capped things at 80x80). If we raise all of the limits too high, it affects how usable PC is for those browsing from slower connections, as well as affecting site usability for those who are on smaller screens. And, if memory serves, the limits were raised quite a bit somewhere around a year and a half ago.