The length of a FW from truck bumper to rear of trailer would generally be 5 1/2 feet (or so) less than the overall length of the FW.
I pull one that is 34. 5 ft. I am very comfortable with it, but it is BIG and it takes some planning sometimes when stopping or parking although it has never been a real problem. I've never been to a camp ground where I couldn't park. Some sites have been so small that I didn't have a decent place to park the truck, but I've always had room for the trailer.
Someone mentioned resale of larger trailers, when trying to sell larger trailers tow vehicles can really become an issue. I've been trying to sell mine lately, alot of people would like to have it but don't have enough truck to pull it. Most 37-40 FWs would require a 3500 DRW, and that could be considered to be marginal.
There are alot of 33-35' FWs out ther that are quiet roomy and livable.
FWIW,
Fireman