He should be able to stand without having his face jammed into the front of the vehicle, and without having to have his tail crammed up against the back door. Since most rides take place during the fly season, either put a fly sheet on him or be sure he has room to use his tail to brush them away. Keeping the trailer sprayed will also help control this problem. When you hang up a hay net in the trader, don't stuff it so full that it takes all his head room away. During the trip it is very important to get your horse off the trailer from time to time.
Many people feel that it is best to unload every four hours or so.
During these breaks the horse should be watered, walked, allowed to graze and relieve himself, and trotted a few times in hand. Horses frequently will not drink while on the trailer, and especially in hot weather, water loss can have serious consequences. Also, many horses will not urinate in the trailer, resulting in discomfort and unnecessary stress. Another reason for getting the horse off is the accumulation of detritus in his airways. A horse must lower his head below the level of his shoulders to cough and clear his throat, and he may not be able to do that in the trailer.