If you have the room, it's nice to be able to open the slides. As far as height, go high enough to pull/back in, but don't go too high otherwise you loose protection. If you get sides the height isn't as critical. Try not to do what I did, had carport built for boat, bought trailer and had to raise it. Now considering a fifth wheel, and will have to raise again.
Steve & Sue
2017 Grand Design 297RSTS Travel Trailer
2017 Ford F-350 Lariat Crew Cab diesel 6.7 SW LB 11,500 lb GVWR package
Former: Crossroads ST26RB