The maximum size is actually now about 100 x 100 pixels. It seems that topic is a bit out of date.
That's really weird
A few weeks ago I tried it when I made my avatar and I had to scale it down to 80x80. I'm ok with this size, but if people think it's 100x100 px and it doesn't work, it could lead to some problems/incomprehension
While reading another post about avatar sizes, Master Durangus pointed your reply here.
So I decided to check this again only a couple minutes ago with the original 100x100 px avatar I made and it was refused ("picture too big").
Also made another one to check and it was refused as well
I even did several tests moments ago, and even with 90x90 px, it doesn't work, I have this message :
"The following errors occurred when trying to save your profile:
* The avatar you have selected is too large, or not an avatar."
So it looks like the max possible personal avatar size is still 80x80 (with the exception of already existing 100x100 px ones in the forum database)