Great question. Gravity doesnt like pointy bits! And the force of gravity is to do with the size and the mass of an object. I’m not sure about the mass, but I’ve heard (and thus worked out) that an object of more than 400 km diameter (tho this varies depending on density) will have a basically spherical shape because its internal gravity field will have equal strength on all surface points.
Interesting one – maybe there’s an equation for it somewhere? I know the dwarf planet Ceres has enough mass to compress it into a sphere – it’s about 1/10000th of the Earth’s mass and about 500km wide, so there are some constraints on the lower limit.