Complex sentence: Proteus is not significantly elongated, but not fully spherical either: it resembles an irregular polyhedron, with several flat or slightly concave facets 150 to 250 km in diameter.
Split into simpler sentences: Proteus is not significantly elongated, but not fully spherical either. It resembles an irregular polyhedron in having several flat or slightly concave facets measuring from 150 to 250 km in diameter.