If g_a is the acceleration due to gravity on planet A, then its radius ra and g_a are related as,
g_a = G (M_a)/(ra)^2 ....(1) where M_a is the mass of the planet.
Similarly, g_b = G (M_b)/(rb)^2 ...
We calculate the ratio of the radii of the two planets. If one of the radii is known, then the radius of the other planet can be calculated.