  Transiting planet surveys like Kepler have provided a wealth of information
on the distribution of planetary radii, particularly for the new populations of
super-Earth and sub-Neptune sized planets. In order to aid in the physical
interpretation of these radii, we compute model radii for low-mass rocky
planets with hydrogen-helium envelopes. We provide model radii for planets 1-20
Earth masses, with envelope fractions from 0.01-20%, levels of irradiation
0.1-1000x Earth's, and ages from 100 Myr to 10 Gyr. In addition we provide
simple analytic fits that summarize how radius depends on each of these
parameters. Most importantly, we show that at fixed composition, radii show
little dependence on mass for planets with more than ~1% of their mass in their
envelope. Consequently, planetary radius is to first order a proxy for
planetary composition for Neptune and sub-Neptune sized planets. We recast the
observed mass-radius relationship as a mass-composition relationship and
discuss it in light of traditional core accretion theory. We discuss the
transition from rocky super-Earths to sub-Neptune planets with large volatile
envelopes. We suggest 1.75 Earth radii as a physically motivated dividing line
between these two populations of planets. Finally, we discuss these results in
light of the observed radius occurrence distribution found by Kepler.
