80x80 and the file size less than 6 KB. I've been wanting to change mine for a while but I can't make any I like meet that criteria. Any way that could be loosened up a little, like up to 100x100? It's no big deal if not. I still like Reno 911. 8)
I regret to inform you that Frequent Members are not allowed to modify their avatar
You'll have to make 1500+ posts in order to move up a level and then have the ability to change the avatar. I'm sure that won't take you long 8) (joking hey!)
The restrictions in place are to ensure the time the site takes to fully load on a web browser is kept minimum. Even today, I consider the interface a bit heavy, so I'd suggest simply performing a resize and cut-down in quality of your new avatar to bring it in line with guidelines.
If you like I can also help you with this process.