I had a 6x12. It was annoying trying to back it in to campsites, etc because I couldn't see it in the mirrors to keep straight. By the time I saw it in mirrors, it was crooked and I'd have to pull forward and start over.
7x14 would be a good size, while not being so long to become a challenge in parking lots - pull through and take up 2 parking spaces.
I'd HIGHLY RECOMMEND a minimum tongue length of 4'. Less then 4', sharp turns become an issue due to width of the truck camper.
Good info in my trailer thread:
Upgrades to my trailer - 8.5x20 Haulmark
