Foostamp - as far as maximum size, you mean for graphics?
I'm not sure is the short answer... I think unlimited, but if the provinces are not sized properly I think castles and units can be too big (or maybe small) and look funny.
As far as provinces, there are some with 1200+ provinces I know of.
My advice would be look at whats out there and see if you plan on making something bigger.. If so.. test with a bad graphcis version. You can see whats out there at:
http://dom3maps.wikidot.com