Radius of a main sequence star is proportional to the mass of the star. If you know the radius, you can pretty accurately get the mass. (It depends some on age and metallicity too, but not that much, as long as it hasn't evolved to a red giant yet.)

Planets, on the other hand, don't follow the rule nearly as well, especially for gas giants. Jupiter and a brown dwarf 80x the mass of Jupiter both have the same radius.