The avatar file size of 64KB too small for you? Me too. But I can't do anything about it. I have, however, found a work around.
When on the avatar selection page, you'll see a section called "Link to off-site avatar". Do that and you can link an avatar of any file size.
It doesn't resize though, so be mindful of the actual dimensions. You don't want to accidentally fuck the forum by choosing a gigantic picture after all.