Often it's not just height though, it's also the build of the horse.
Shay-la is 5'9" and I am 5'6" and we both look pretty ridiculous riding Justus who is a 14.1hh Welsh cross pony but she's as narrow as they come. However, Flika is only 14hh but she's a Quarab, so even in picture, we actually look totally fine on her because her barrel is so huge, it takes up our leg. I actually feel smaller riding Flika, then riding my own purebred Arab who's 14.3hh (3 inches taller) who's also narrow.
My Paint filly is 15hh right now, and I feel that's a perfect height for me because she's stockier then my Arab. I'm a bigger girl, so I look silly riding narrow horses even if they're actually taller then the stockier horses I ride. I enjoy riding anything that's between 15 and 16hh. Any bigger then that, and I feel like I'm perched on top of a mountain :lol: That's what happens when you ride little Arabs your entire life!