We ordered a 6x10 Stealth Liberty with the customized height. I think mine is 6 1/2'. I'm not quite 6' tall and can stand up in the trailer just fine. I smoke my head on the rear ramp door every once in a while, because I have to duck down a little to get into the trailer. Just remember the air vent on the top will also take up some space.
We used to have a 5x8 and raced sprint karts with the wide wheels and we would have to take one wheel off to get it into the trailer. A dirt kart will fit into a 5 just fine though.