I don't know what size you are, but I'm a gigantic mutant gorilla. 6'4" and a fat bastard to boot.
Because of that, I'm always very aware of this thing called "elbow room". My little, old, class B camper van is built on a standard length Ford full size van with a 24" fiberglass bubble top. 6' wide at its widest point, 6'2" inside height (at its highest point) and 13' from dash to back doors. Its got all the "self contained" stuff - sink, toilet, shower, fridge, stove (with oven, no microwave), furnace and water heater.
It has a 6'4" couch lengthwise behind the driver's seat. Across from that, behind the passenger's seat is a clear space of about 4', and then the galley. The toilet is rear center (pun intended) and the shower in the rear corner, behind the galley, passenger's side. The aisle between the couch and the galley cabinet is 16" - and it is NOT enough.
When sitting on the rear half of the couch that is across from the galley, I cannot stretch out my legs. When sitting on the front half of the couch where the open space is, I can. When spending days in the camper, this becomes a major issue.
When standing up, the 6'2" height is not so bad - I barely notice it. When trying to turn around while standing up, I REALLY notice the lack of space in the aisle to put my feet. That clear space, which is roughly 4'x4' is CRITICAL to me. I can stand up, change clothes, put on or take off my jacket - there is enough elbow room.
Now, that's with just 1 person in the camper. With two, it would quickly become intolerable, as we would be regularly poking each other with our elbows and stepping on each other's feet.
Rig size is always a tradeoff. My little van camper is only 2WD, but its still a truck and it goes a LOT of places. It's fine for one person. If I was going to spend a year in a box with another person, I'd want more elbow room. A bigger truck would give me that, and still get to just about anywhere this one can go. If it was 4WD, it would get to even more places than this one.