Hi Coco, there is not size limit on your image. PrestoPhoto will show a thumbnail of it in your gallery. It can then be clicked on an it will be on its own page large (PrestoPhoto automatically sizes to a viewable size) with options under to view it in different sizes.
Kim My Creations Canon 40D, Canon 28-135mm IS lens, Canon 300D, Canon 18-55mm lens, CS3
Since I already had some galleries out there, a couple from books I made, and others I was just using as galleries, I'm assuming my space for a "Community" gallery is pretty limited. Guess I'll just play it by ear, and if it fills up I'll look into purchasing more space.