Definition: An avatar is something used to represent you in an online environment. This is usually in the form of a small picture. Usually it is something that you can upload yourself, however sometimes it is something you "build" on the site to represent you. Other times you choose from a list of possible icons to represent you as your avatar.
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