A mass-radius relation is an equation relating a star's mass with its radius. An example:
log R = 0.003 + 0.724 log M
This is a least-squares fit for data covering main sequence stars over a variety of masses and radii, from data derived from eclipsing binaries and radial velocities.
Better fits are found by limiting the interval of mass values, such as varying the constants based upon the mass. A plausible break-point in the model would be the point between the masses of stars with significant convection and of stars without.
With the mass-luminosity relation and color index, this relation can be used in rough estimates of stellar parameter determination.