BILL– OK, I am staying totally away from the “size matters “jokes. But when it comes to choosing a horse, particularly if you’re paying money for it, as you take into account all the suitability factors, size should be one of them.
My advice is pay heed to the Goldilocks Syndrome. The horse should be big enough that you feel comfortable on him and he fills out your leg. That means not so narrow that you can’t find his sides with your calves and deep enough in his body that your feet don’t hang below his belly. In terms of a horse’s ability to carry weight, you won’t hurt a smaller one unless the disproportion is extreme. For some jobs smaller is better. If you are constantly on and off a horse while repairing fence lines or working cattle, big just gets you more tired. On the other hand if you are jumping, and your horse clears the fence but your toes pull the rails down, you probably need a bigger horse!
In the dressage world for a time very large horses were in fashion. It was said that judges wouldn’t give smaller horses a second glance. As long as the horse you pick is suitable, I don’t think that premise holds water.
Having owned an 18 hand horse whom I competed in the FEI, I would not do it again. The 60×20 arena can look awfully small when you’re trying to fit a line of tempi changes on that diagonal! Besides that is the question of how much mass you’re going to want to move around all the time. If you are 30 years old and stand 6 foot four, that’s one thing. If you are petite or like some of us getting a little longer in the tooth, do yourself a favor. 16.3 or 17 and is plenty big for most riders.
One last thought about very large horses, particularly ones who also move big, is the increased stress that mass puts on joints, tendons, and ligaments. There is no guarantee that a smaller horse will stay sound longer, but I would not go looking for trouble by choosing the flash factor over common sense.