As far as I read the sfml code, sfml seams to load the biggest verion of a required character, whats basicly a good idea, but it leads to bad results, because OpenGL can't ever do nice scaling(as it does not know you are using a vector image source)
I think it will look better, but this will also havily depend on the font file you use...hmm I'll try out if I can really do better(thought unicode would be more interesting)
have fun!
EDIT: I tested a bit and for DejaVu I get a limit of 10px, bellow you cant read any small letters so I'd really suggest to use a bitmap font for anything smaller.