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 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.