There was a long tutorial I wrote about stuff which resolution and image sizes were discussed.
[Award Winner] Bitmapped Images - The technical side of things explained.
I think that the rules don't say what size you need to make for the challenges but something about screen size is fine. Some people make maps bigger than can be seen on the screen because they might print them out and the DPI required is higher for a printer than most screens. That's covered in the tut. Also we might not be viewing the map at 1:1 ratio and I use a zoom browser for my maps which does not have a fixed view scale.
If your still in doubt then do something about the ole 1280x1024 type size which should be fine.