Well I am 15 years old, and theorioticaly I should stop growing, because the tallest woman in my family is 5ft10, while the rest are only around 5ft5 etc.
Anyway I am 5ft8, and 57kg (125.63lbs), and my horse is a fine built 15.3-16hh Thoroughbred. After research I have discovered that a horse can carry 20% of its body weight, so my calculations indicate that Harry (450kg approx (992lbs)) can carry comfortably a person weighing 90kg (198.42lbs)- Although I say Harry can only carry 15% because he is finely built, but this still only indicates that he can carry a 67.5kg person (148.81lbs)!!!!!
This means that as long as us tall riders can keep our weight down we can ride basically any horse or pony (as long as they ain't much shorter then 14hh, and squarley built with large barrel to take up leg)!!!
I am extremelly happy, and just thought i'd share my research with everybody out-there who worrys about being too big for their horse.
I hope I haven't bored you completly with my techno talk, I tried to keep it descrete LOL
P.S - here is a picture of me (5ft8) yesterday on my 15.3hh TB. PLEASE ignore my long reins and awful position, we just came back from XC schooling and he was a little fired up, was trying to hold him still for the camera LOL
P.P.S - here is also another picture of me on Harry a year ago in action, to show my length of body on him etc (sorry mum sucks at taking photos so I hardly have any up-to-date one, my riding has improved alot)