How big does a mass have to be before gravity

Sounds like a good exercise for learning/practicing what you know. Compressive strengths of minerals and mineral composites (rocks) run ~ 30kpsi, or 200MPa. Densities ~ 3000 kg/m³, or sp. gr. ~ 3. Pick your units. G = 6.67 x 10 ^-11Nm²/kg². How large, volume or mass wise, can an accumulation of material get before surface structures crush under their own weights to less than some upper limit of radius you choose to allow for relief?

 

There is apparently not a bright line https://en.wikipedia.org/wiki/Hydrostatic_equilibrium

 

There shouldn't be a bright line - astronomical objects have different compositions, and their weight and strength depend on these compositions. But the transition region in the solar system seems to be in the 200-500 km ballpark. Icy moons tend to become round at lower radii than stony or differentiated asteroids.

 

The usual mentioned numbers for icy bodies is more like 400-600 km, as the Wiki ref says, while stony ones lies over that range. But YMMV.