I don't think there's any size, but as small as possible while keeping the best quality (i.e. no funky resizing) seems good to me. I'm not even sure why we have that random Luigi victory smiley. If you feel like adding in the others you can, though it's not overly necessary, heh.
Lol I just animated a Yoshi one from the victory screen, just trying to figure out how to save it and then I'll PM it to you and we shall see if it works.
