This came up in the last Brian Cox series on the BBC, Solar System. Asteroids are large lumps of rock in all shapes but often looking rather like a potato, with one axis larger than the other. When rocky objects get to a certain size, they have so much mass that gravitational forces can pull inwards enough to make the rocky object spherical. Then we can call it a Dwarf Planet. The Potato Radius is the smallest radius of such a dwarf planet. It is somewhere between 200 and 300km.
