HP, I don't have a good answer just yet, but I'm working on it. The way the forum administration works is that I can set a max size, but I also set "Resize through HTML" so that if anybody puts in one larger, they still get the avatar, but it resizes so we don't get some huge image. The problem with the logic on the board is that it applies the size to anything, even if it is smaller.
The easiest way right now to fix is to upload the avatar instead of using the URL link. That will upload the avatar in its native format as long as it's below 120x120 and retain the native size.