For very large ships, I usually just make a block diagram of the different areas of the ship. Then I have a handful of interchangable small maps (you can build them from the bits provided in the SW Starships of the Galaxy book, if you have it) of important areas, for when combat pops up, or detail is needed.

Having a deck plan for even something like a star destroyer is neat, but is about the equivalent of having a map of a small city with floor plans for every single building in the city. It's a lot of work and a pain to keep track of.

As much as I've looked, I've never been able to find actual deckplans for very large ships.