LOL I thought in Photoshop things are raster from the start? Hmm when it comes to sprites, Lysa's best advice would be select certain pixels and shift them as you'd like rather than resize automatically. Well, that's if you wanna shrink em. If you wanna enlarge them, then the best method would simply be to remake/trace a larger one. Yeah, sprites are difficult to handle...
As for the size of the signature, I'm thinkin 100 or 200 px in height. Proportions being 1/4 (width being four times the height, but no strictly as much, just whatever you're comfortable with).
XDDD I think I'd stick with beardless, since that's the most popular one as of now. Yeah that art would rock!