Bed length varies by manufacturer and model year. There are generally speaking short box, standard box, and extra long box versions with lengths of 5.5', 6.5' and 8' respectively.
The overall length of the truck and the wheelbase depends in part on the length of the box but equally on the cab type, regular, mega/double/super, or crew cab. With a crew cab I would not want the long box as that makes for a very long vehicle with a terrible turning radius.
There are sometimes differences in the fuel tank size with the different bed and cab configurations and differences in the availability of aftermarket replacement tanks for the OEM one.
Test drive different configurations before buying anything and if you have a local RV park in your area go there and talk to people pulling 5th wheel trailers.