Mine is 100x97, even though it SAYS 80x80 is the max size, I just put in the proper dimensions and voila.
Try it out. Perhaps that's a holdover from an earlier version of the forum. (no, it wasn't part of the ranking system, either. Mine's been this size since I switched to it from the jiggin' Black Mage, back around november 8 or so. I've seen several long avatars -- dumbsioux is the first that comes to mind).
-Tempest out.-