Asteroids and smaller celestial objects tend to be odd-shaped, whereas planets are always spherical (or slightly oblate due to their spin). The reason obviously is that for sufficiently large masses/sizes the gravitational force overwhelms any resistance of even rocky materials to maintain a shape other than a sphere (which minimises the gravitational energy).
The characteristic scale at which this transition occurs depends on the material and is different for ice and rock as mentioned in this answer (to a related question) which also links this useful Wikipedia article. I wonder whether this limit can be estimated from simple physics (involving material constants for ice/rock).
