Don't associate length with weight. You need to look at the 5th wheels UVW and pin weight. Add whatever you think you'll pack and thats what you can tow. A 36' DVR Mobile Suites weighs almost 16,000 dry. A 36' Grand Design Reflection weighs 10,500 lb dry. There are many variables in between those two.
Look at your door weight sticker and tow rating. JMO but assuming you have a 4x4 I'd look for a loaded 5er of around 14,000lbs. That would give you a 2800+ lb pin weight. Add in all the other stuff you'll put in the bed and you be getting close to the max payload. Main thing is to check the payload sticker on your door jamb. Thats the weight you can put in the truck , like passengers, camp stuff, 5th hitch and the 5er's pin weight.
