I used the TS 4x8 trailer and built mine to fit inside the frame. Or should I say still building. I used the Generic Benroy plans and like MrBuzz, I didn't do everything as the plans show.
You will want to look at it as you build to see were you like the placement of everything. I put up the walls and bulkhead (bulkhead was positioned to get the length I wanted for sleep area) and drew the foot well and cabinets right on the walls to get a feel of how much room I would have.
I also opted not to have lower galley cabinets and smaller interior cabinets. I made up for the interior cabinets by building a tongue box that you access from the inside.
I would recommend that as far as the little shelves here and there and storage in the cabin, if you can do without, it gives you a much more roomy feeling.
My boys and I spent two nights in the trailer a week ago, out in the driveway. So thats a 4 year old, a 7 year old, and me at 5' 10" and 220 lbs sleeping in a 4x8 with no bunks. Not bad. Slept like babies.