Our last two trailers have been 8' tall. More important for air circulation in warmer climates, but I would not go below 7'6" for any horse over 16 hands. You need to know where he comfortably holds his head. Horses don't trailer like they are doing western pleasure and use their heads to balance from front to back and side to side while swaying in a trailer. Another thing...7 foot high trailers (or any actually) measure to the roof itself - not the ribs/rafters that support the metal roof, making them really only 6'11". I measured one to be sure!!! Your 17 hand horse would not be happy in a 7' trailer!!
I would go 7'6" for that tall of a horse.
I have a 7' trailer for my 16.2 big boned horse and it's fine for him, but he trailers like a dream. But if you can find and afford a 7'6" I would do that.
My trailer is 8 ft tall and my friends 17hh horse who is a bit of an idiot about loading/trailering fits in there very well. He also carries his head very high.
ETA: She recently was thinking about a 7ft tall trailer and I agreed with her gut feeling that it would not be a good idea. Also, she is very tall so future horses are likely to be on the bigger side, so decided to wait for a taller used trailer to come along.