  Recent studies regarding the habitability, observability, and possible
orbital evolution of the indirectly detected exoplanet Proxima b have mostly
assumed a planet with $M \sim 1.3$ $M_\oplus$, a rocky composition, and an
Earth-like atmosphere or none at all. In order to assess these assumptions, we
use previous studies of the radii, masses, and compositions of super-Earth
exoplanets to probabilistically constrain the mass and radius of Proxima b,
assuming an isotropic inclination probability distribution. We find it is ~90%
likely that the planet's density is consistent with a rocky composition;
conversely, it is at least 10% likely that the planet has a significant amount
of ice or an H/He envelope. If the planet does have a rocky composition, then
we find expectation values and 95% confidence intervals of
$\left<M\right>_\text{rocky} = 1.63_{-0.72}^{+1.66}$ $M_\oplus$ for its mass
and $\left<R\right>_\text{rocky} = 1.07_{-0.31}^{+0.38}$ $R_\oplus$ for its
radius.
