I believe weight on the pin is the most critical component. Since about 6 feet of the camper is directly over the tow vehicle's rear axle, the pin weight (15-25% of camper weight) keeps the rig on the road.
As to length I think adding the 6 feet to the wheel base calculation would be safe, but have no data to back it up.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW