Good practice when choosing an avatar is to keep your image small. Some sites will post the maximum dimensions allowed for an avatar. If you can't find it posted, look at what the other members are using and let that be your guide.
As a programmer, you want to limit your user's avatar size so that it doesn't distort the size of your layout, and to limit the use of bandwidth and the load time of pages for people with slower connections. If you're interested in letting users upload their own avatars to your site, try this script: upload with php