Wikipedia: "The traditional avatar system used on most Internet forums is a small (96x96 to 100x100 pixels, for example) square-shaped area close to the user's forum post, where the avatar is placed." 100x100 is the main size for an avatar. You can't change it. Other people may ask as well to increase the avatar size more than you asked, but that's impossible since there would be no need for a limit if the limit keeps changing.