I believe you are looking for hydrostatic equilibrium. There don’t seem to be good answers for this online, but according to Robert Black on this Quora post:
There isn’t a minimium per se but the generally accepted number for a mass to form into a sphere under its own gravity is 1/10,000th the mass of the Earth or 600 quintillion kg. As for size, it really depends on the composition of the body. The numbers are generally accepted to have a diameter of about 600km for a rocky body.
A quintillion is 1 x 10 to the 18th and Phobos has a mass of 1.0659 x 10 to the 16th kilograms and a diameter of 22 kilometers.
Phobos is this big and still not round? Uh, what was the name, the size where stone behaves like a liquid. Well, Phobos doesn’t have that yet?
I believe you are looking for hydrostatic equilibrium. There don’t seem to be good answers for this online, but according to Robert Black on this Quora post:
A quintillion is 1 x 10 to the 18th and Phobos has a mass of 1.0659 x 10 to the 16th kilograms and a diameter of 22 kilometers.
Yes that, thanks!