Depends on the horse! Cobs or heavyweight hunters and heavy horses can take most people, ask the riding school or owner and tell them how much you weigh. I know someone who weighs about 16 stone and they ride regularly, admittedly its a big b*gger, about 17 hands.
Obviously the fitter you are the easier you will find it. My problem is I weigh nearly 12 stone, but am only a short *rse so although I need a horse that can take my weight I have trouble getting on them without a mounting block!