The frigate is the ship size one step above a corvette. By default a frigate has twice a corvette's hold and a larger tank, and it is slightly slower. Its base structure is higher than a corvette's, so even a light frigate is no weaker than a corvette of similar mass.
The freight ship, the constructor, the surveyor, the escort, the scrapper, the lander and the clipper are frigates. The Outpost and the Workshop are frigate size, the smallest station size. A dock takes ships up to its station's size, so every frigate fits every station's dock.