When you begin a new design, you’re asked your ship’s size and the measurement unit you’re going to use (meter or foot). AFAIR, later you can change units from the FILE dialogue box.
Each part of the ship must be drawn on its own layer, this makes things much easier. If you’re really puzzled with isolating a ship’s part (when printing, etc.), you can delete all the ship but the part you’re interested in, and save the project under an other name (do not erase your main model! 😉 ).