Well, technically you could go higher than 1920x1080 HD resolution, if your monitor supports it, but I don't see why you'd want to.
As for sprites, they're 32bit, iirc (24 bits for color info and 8 bits for alpha). Someone correct me if I'm wrong.
Oh, and before you get all graphics-happy, you should read up on optimization on the wiki:
http://apps.sourceforge.net/mediawiki/c ... ation_tips
Just because you can make huge sprites doesn't necessarily mean you should.