For rocky bodies (asteroids, dwarf planets) it's about 300 kilometers
Based on looking at Solar system objects, it's actually not that clear. The reference that is often used is that Ceres is spherical and Vesta is not, so therefore the boundary must lay between them. But that is not a good reference to use since Ceres' composition is more similar to an icy world than a rocky one (it is nearly the same density as Pluto, as well as other icy bodies comparable to its own size). So although a rocky/metallic body may be able to be a sphere at Ceres' size, we have no proof of this. As I said in the other thread, the smallest spherical rocky/metallic body in the Solar system is Europa, and the boundary almost certainly lays far below its size.