It's cramped but you can fit two people on top of each other with that scale =P I just imagine there's very low ceilings in the ship.
For the floor plan stuff, I usually do google searches for floor plans of other sci fi ships, and then cut out bits and pieces as I need them (for things like chairs, beds, consoles, etc.) I do the walls and windows and such by hand just using the paths/stroke tool and a bit of feathering and blurring with a lot of layers. I took things like floor tiles out of the other ship floor plans and then turned them into gimp texture files so I can fill an area with a continuous floor texture using the bucket fill, makes it easy to quickly plot down floor textures throughout the ship.