Researching the necessary technologies will automatically unlock the ships. Which technologies those are depends on the ship-size and equipment used for the ships. If you don't remember what they are, then clicking on the hull-unlocking techs in the tech tree (Interstellar Construction, Medium Scale Building, Large Scale Building, and Massive Scale Building) will tell you. Those techs will show you all the ships of this size you've created for the race you're playing, and what techs they require to be build (except for Miniaturisation, but it will indicate if you have enough).