this topic has been discussed at lengths on the forum before, I always think horse's can carry more than you'd expect but it all depends on the balance of the rider. I would say a 15hh horse with 8-9 inhc bone would easily carry 11 stone, though if the rider has no balance it might be kinder if they were lighter...I do expect some controversy over this
Fitness also has a lot to do with ability to carry weight. An unfit/ fat horse just about carries themselves, whereas a superfit horse has the muscles and can cope much better.