I guess I'm not the only one... HEre's what happens to me: I design a ship and from the begining I know I want a jump drive on it. I add everything I want and then I start creating a new jump drive to fit the size of my ship. I commonly need to design 3 jump drives to find the right one that isn't too big yet big enough to fit on the ship when taking into account extra room for crew and armor.
It would be a nice addition if the game could tell what the minimal drive size should be to allow a given ship design to jump in the design window, or if the project window could tell how small a ship mounting said drive has to be to be able to use it.