Reading "its radius is x km" would mean nothing to you if you'd never experienced spatial extent directly, whether that be visually or just by moving through space and existing in it. You'd need to do exactly what is being said in the paper, read about thousands of other roughly spherical objects and their radii. At some point, you'd get a decent sense of relative sizes.

On the other hand, if you ever simply see a meter stick, any statement that something measures a particular multiple or fraction of that you can already understand, without ever needing to learn the size of anything else.