I don't think the problem is the trailer size once the horse is in, I think the problem is loading the horse in the trailer. In that case load your horse first! Don't get fustrated is the key thing. Reward horse with scratches, pets, treats, etc., but don't bait him with food! That is a nono. Have a lunge whip (or dressage whip - I like both for different uses). Make him work outside the trailer and not when he gets in the trailer.
“Good things come to those who wait… greater things come to those who get off their *** and do anything to make it happen.” - Unknown