It depends on how you save it. If you save it as a template, then the template (design) is usable no matter what items/equipment/modules/etc you add to the ship.
However, if you just save the ship with weapons and all, there is a good chance that you will not be able to build that ship for a while since there are many techs which reduce mass of various components, allowing you to fit way more stuff on your ship. With everything unlocked, your ship design allows very close to the maximum amount of space. Of course, in a regular game, it is unlikely that you have as much space to use... thus the ship may not be available. (unless of course you design a ship with plenty of room to spare).