I don't think that is shown anywhere,(Radius). Except for if you land on a planet, the distance it gives you is the distance to the center, IE Radius. And height above land is shown on bottom right.

If you look in the view that shows the orbits in the system (the one that allows you to speed up time and watch how the planets orbit) and select a body, it will show you the radius in km. And actually, the radius of the planet doesn't necessarily correspond with the mass, I've found a number of planets that only had about half the Earth's mass yet were actually larger than Earth in terms of their radius. This is to be expected, as the composition of a planet obviously has a large impact on its mass (planets made of heavier elements will be heavier).

It's worth taking a look in that 'orbit' view, it also shows a planet's distance from their parent star (or a moon's distance from its parent planet) in km as opposed to AU.

You also have much higher densities on heavier planets due to gravitational compression.

Ah there it is. Found the planet radius. Thanks very much!