The avatars are 100 by 100 pixels. A lot of information about the forum is in this post in announcements:http://www.badmovies.org/forum/index.php/topic,113524.0.html
Stating the max size on that page is a great suggestion. I'll take a look at the source files and see if I can't find where to modify them so it displays there.
The editing posts trigger is having 100. It's likely we could reduce that and still deny the spammers their due. Let me take a look at some statistics though, because the limit on editing posts came about due to some sneaky spammers. They would post normally, but go back and edit their old posts to add the spam links. The hard part is balancing being able to edit posts against how old of posts someone can edit. I'd rather have no limit on how old of a post can be edited.