Ships should have enough space to host their crew or else will become non operational, you won't be able to build such ships although your design will be saved.You can create designs with unmanned ships, but the options are limited as advanced hulls and modules require staff to operate them.You can design ships without propulsion, and you will be able to build such ships. But those ships won't be able to travel and will stay in orbit of the colony that built them.You can edit a ship design as many times as you want if you do not have already built or queued any ship of this design. Likewise you can delete a design.