What you need to do is wait for one of those forum staff go loco days—when the staff “go around” dishing out super sized avatars to online members. Once you’ve got yours, all you have to do is log out & never return. If you’re lucky, the staff will accidentally overlook you when reverting the avatars, and you’ll be one of the few members to have a 100 x 100 avatar to its name.
It would probably be some picture of a leather-clad Donald Duck, but hey, at least you’ll be Unique.