What size horse should I be riding?

I would definitely say you need a taller horse, your weight is fine, I ride a 14 hand pony who can easily take my weight but he is also a little small for me I'm 5' 8, and my legs do the same thing, I only ride him because he needs schooling, but I prefer riding a tall horse because of it. You should be anywhere between 15 and 17 hands, though going to tall can be a problem to, you should be fine though.