My couch is 75 inches long, 12 inches high. The width varies from 15 at the wheel well to 18 near the rear. The front part of mine holds the secondary battery and a little storage. The middle holds some but right in the middle there is a 5 inch(?) wide board to support the couch that you have to maneuver around. I have some pictures on my website but can send some privately if you want.
Remember the turnbuckle ports are in there too and you need to be able to access them to make sure they are tight enough.