Well there are many ways to do this; if you are in a space craft, the angular separation of the limbs of the planet can be used to get the radius when the distance between the observer in the spacecraft and the planet are known.

Another way is to use a” planetary transit”. This means that, if a planet crosses the face of its parent star and this event can be observed, then the SLIGHT decrease in luminosity of the star that is being occulted can be translated into an estimate of the radius.

The radius and the mass of a planet are very important in order to calculate the density of that planet. The mass of the planet can be calculated using spacecraft tracking; then, the ratio of mass to radius cubed is the DENSITY. Density is the most critical variable to know in order to understand the evolution and nature of the planet.