I'm 5'11" and ride a 14.3hh grade gelding. He's built like a tank so it works okay, even if we do look a bit ridiculous (especially jumping when my face is over his ears
) . I ride my 15.3hh CWB mare no problem, again she's a thick girl and eats up the length of my leg. Then again, I look best on my 18hh CWB gelding...he doesn't look large nor do I look too small.
It all depends on the build of yourself and the horse more than heights, imo. If the horse can comfortably carry your weight there is no reason why you can't or shouldn't ride it!