650 x 500 is a good size. I usually do 600 x 450 but I think they allow a little larger.
edit:
This just in:
Images. Please resize images that you post or attach to a thread to no larger than 750 pixels wide. If you wish to have a larger photo then make a link to the photo that people can click on. This will save the bandwidth and page load time each time someone views the thread.