your going to want at least a 7 ft cieling, 8.5 wides are maybe the better option too here
then you have more room to walk squeeze past on the sides
I would think a MIN of 19+ ft long, but longer the hauler, the more space you will have for other things
you looking for a bumper/class 5-7 hitch pull, or looking for a 5 th wheel??
have always had good luck with Haulmark trailers,(currently have a Worthington all aluminum, but for sleds not utv's, not tall enough what I have, its only 6 ft tall!)
but these days, so many good one's out there, just stick to an all aluminum if you can, way less to worry about over time! Triton's are like cream of the drop, and prices reflect so too! but a ton of options out there to be honest
have a look at this for a simple idea, a 1000 poineer shouldn't be any bigger than a 4 seater RZR