Our trailers are 6'6" and it's plenty, we have have some taller horses 15+H.
One thing I have noticed, if you back your horses out of the trailer they have the tendency to raise their head to help them look and place their feet to step down. I have seen horses hit their head doing so causing some anxiety to step down off the back and even their head higher. If you back out you might want to consider a taller trailer.
I turn mine around and let them walk out(not jump) and they will put their head down to look where to step out so I don't have an issue with them hitting their heads.
Not saying that unloading one way or the other better, mine can unload both ways, but it might be something to consider for trailer height depending on what you do.
Happy trailer shopping :)
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.