How big is big enough?

To me, comfortable seating, the size of the bathroom and shower, insulation, etc., are all important. If you go too small, there's not room for a good chair (RV seating is generally not "good" seating). A shower that's too small to turn around in is not tolerable for any length of time. Insulation... check out Northwood Manufacturing (Actic Fox and Nash lines). In travel trailers, they are some of the best build on the market. There are people who full-time in truck campers and B vans - but are you willing to be that confined during a rainy stretch? All things to think about. Oh - and you'll almost certainly need to replace, or at least supplement, the OEM mattress, too.

Rob