Okay - this has been fixed. The site will now resize it for you so you don't have to worry.
While we're on the topic, I know a lot of people know this but for anyone curious... if you want the biggest avatar possible, you should use a square picture. The site will reduce your picture to 100 pixels at it's largest dimension, but it will keep the proportions. Thus a square picture will end up 100 by 100, whereas a rectangle will be 100 by something less than 100.