As mentioned by other posters, horse rider fit is influenced by: discipline, horse's conformation, rider's conformation, and the amount and level of work you expect to do.
It's impossible to judge from a rider just posting their height and weight and the horse's height.
In general, gaited horses and western discipline horses can carry bigger and taller riders because 1.) the horses do not move with a long stride or lots of suspension and 2.) the rider does not ride with their lower leg on the horse.
In hunters, jumpers and dressage horses, the rider needs to wrap their lower leg on the horses barrel, or at the very least have contact between their lower leg and the horse's barrel. An experienced or skilled rider may adapt their postion and stirrup length to school a smaller animal but they compromise their security somewhat doing so.
In hunter seat equitation, and to a smaller extent, the collective mark for rider's position in dressage, that the rider "fit" the horse is critcal in order to demonstrate correct, secure position.
For example: I have a 14.2 hand foundation bred QH that's built like a brick with legs. He was ridden by a large man (5' 10", 240 pounds,) in rodeo events successfully. I am 5' 7" and plump. I trail ride this little guy but only jump him lightly. He takes up my leg fairly well but it would be easy for me to unbalance him with my upper body. If I wanted to seriously condition him for fitness, I would get a smaller rider to ride him part of the time or pony him on alternate days as I think packing my weight six days a week for extended periods at a long, swinging trot would be too hard on him.
Last edited by maura; 12-17-2010 at 12:46 PM.