It really depends on the horse. My family has an H&S stock trailer that originally had a 6 foot ceiling. We went with the stock trailer, since we can't afford to have a separate trailer for the horses and the cows. My sister's 14.3 hand QH flatly refused to load 9 out of 10 times (NOT a training issue), and my 14.2 mutt wouldn't get on without a ramp (we actually made a ramp for him, the trailer is a step-up). It got to the point that we were ready to try about anything, so dad chopped the roof off of the trailer, added square tubes to the ceiling supports to raise the roof up another 6 inches, and bolted the thing back together. Instantly loading horses! The trailer looks like it came that way, and it's sound as can be. I'll have to make peace with photobucket and find some before and after pics...
Anyway, moral of the story, it depends on the horse. My 14.2-ish guys wanted another 6 inches over the 6 feet to feel comfortable.