Well, we've been practicing in the small space that a 4' would provide...and finding it STILL isn't enough room. We have king at home...and that is plenty, and on a queen, we begin to have difficulties. Unfortunately, we are both sprawlers
Going between a queen and a king would likely be perfect. Just wide enough, we don't bump elbows and knees as often...and long enough, I don't have cold feet (6'1"--7' when I stretch out).
The monstrosity would be pulled by a Ford Expedition if it were built. We bought it to initially pull a big enough pop-up, but then ended up going in the realm of teardrops.