New Ginormous Avatar rules

Actually, it's 150x150, and it tells you.

If you have a custom avatar and want to keep it as it is, leave the fields below as they are. Use Custom Avatar You may upload a new custom image using the controls below. Option 1 - Enter the URL to the Image on Another Website Option 2 - Upload Image From Your Computer Note: The maximum size of your custom image is 150 by 150 pixels or 117.2 KB (whichever is smaller).

if you create a 150 by 150 picture it doesnt fill the whole circle, in my experience... my pictures i use as avatar's are all 600 x 600. perhaps its just me tho

If you upload anything larger, it downsizes it so that the longest dimention is 150px. Maybe the image you're using has a border? It's also possible that it's something with the resize code if your image takes up too much disk space.