its width can be no greater than 80 pixels, the height no greater than 80 pixels, and the file size no more than 6 KB.
If you need to size it down, open it in ms paint, or http://www.getpaint.net
(completely free and works similar to Photoshop.) Then you can move the boarders and shrink the image to the desired specifications.
Avatars must be hosted on an off-site server (like photobucket, or your own website) or alternatively you can pick from the preselected gallery of images.
Obviously keep it tasteful as well.
If you need further assistance let us know.