Good size but not huge, just big enough to get over higher obstacles but small enough a short person (4'9-5'6) can get on without much difficulty.
If there isn't already a breed like this available, I'll just go ahead and post what I think would work best
Thoroughbred: excellent speed, decent stamina/endurance fairly beautiful, good jump ability, too tall
Dutch warmblood: good speed, ok endurancew/stamina, very beautiful, too large
Welsh pony: OK stamina, ok speed, Good jump skills, ok beauty, just tall enough, but not quite the right width
Arabian: Excellent speed, excellent endurance/stamina, not the best jump skills, very beautiful, about the right height and width
Backcross thoroughbred and Arabian, another warmblood, and add some morgan. For a bit of strength and a little smaller height