I believe the best option is the 7' wide x length. From Triton, it's the PR-247, I think. Sleds have to be put in off-set and you put them in the trailer in the same order you take the out the other end. No room to be side by side or pass by. IMO this is a small price to pay.
With a 6' ceiling, it is roughly truck height or slightly taller. Fits into same wind tunnel as the tow vehicle, so low resistance. Low weight if all aluminum. Can't wait until I can upgrade to one like this.
If you are looking for 4 place and with long mtn sleds, you will need a 24' box plus the typical 5' V nose. Total length is usually 31'. Those that have them, love them. Those that don't, want one !