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
There is a 200MB limit for the gallery - which is huge, but if that isn't enough space you can purchase addtional.
If you want to upload a full resolution photo you can - and by the way, PrestoPhoto not only prints beautiful photo books they also do 100 year archival prints too.
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.