Avatars must fit the following size regulations: No bigger than 400 high x 300 wide. Here are some sites to shrink your avatars if they're too big then post in the avatar request thread to be re-upped.
GIFWorks.com! Free Online GIF Tools
Resize Images online
If you think your avatar is too big but are unsure, post here asking and I'll tell you. If your avatar is too big and you ignore this I'll give you a few days before I remove it, though before I do that I'll send a warning.