:As I already said in my edit summary, the point is that Proteus is larger than 400 km and yet is clearly not spherical. The 400-kilometer cut-off comes from Michael Brown's point that everything known above that size is round (and hence in the Kuiper belt). Yet Proteus is larger and not round. Its exact size is not interesting then. People specifically interested in its size will refer to the infobox anyway, where the best known value should (and is) listed.    
 :As for the crater, the only source that was on its article was Gazetteer, which does not list a size. What's your source?