How tall of a trailer do I need?

if its just one time the 7 ft would be a tight squeeze maybe a lil uncomfortable because my riding instructor was advising nm dad on what sized trailer to buy for my qhs and she had a 16.2 through bred and she said there needs to be 2-4 feet above the head and if its a 2 horse trailer than remove the divider make sure that there is enough room for not only the head also the whole body all around front and back hope this helpd