It’s not necessarily height of the person that is the issue - you could be 4’11" and still be better off than someone who is 5’8" with shortened arms. It’s the ratio of torso:arm length. Long waist plus short arms makes for a challenge in arm/hand position plus length of rein issue. People with SHORT ARMS are at high risk of having arms that are straight at the elbows which provides tension to the horse, or alternatively an option for the horse to try bit-related evasions. Shorter LENGTH OF ARM requires a longer rein, and much stronger core and seat to ride the horse through to contact to correct the situation and these riders also have to be aware of their horse at all times (to nick evasions, naughtiness, etc. in the bud).
Another note: A person who is 5’2" looks much better on a smaller horse (say 15HH) which may fix their reach/arm/elbow problem. A person who is 5’6", may have the same length of arm as a person who is 5’2" but who CANNOT always ride the smaller horse because of their overall height. In other words, for example, they have the arms of someone who is 5’3", but a body of someone who is 5’7". I am 5’5", but my arms are the length of someone who is about 5’2". I look ridiculous on a 15HH horse and feel ridiculous too, like I’m perched on a cliff edge. I’m most comfortable on something about 16.2HH but then I have to be right on top of things with regards to my reins, seat and legs. I reject riding horses > 17.2HH because I understand fully my physical limitations/restrictions.
I hope people can understand. It creates a unique challenge that only a short-armed/longer-bodied person can comprehend fully.
Judges who don’t understand my short arm problem always state my reins are too long and knock my scores accordingly. Alternatively, when I ride the next test with that same judge with the reins being the correct length, they then turn around and state my elbows are “too straight” and rigid and knock my riding score again, down to a 6 or 7, maybe a 7.5. Go figure. Interestingly, those same judges will also comment on my ability to bring the horse together on the bit, working correctly over the back from behind into collection. (Which is due to the fact that I have a very powerful core and solidly independent seat and I work my horses through my seat and legs at all times - thanks to the pounding, demanding, insistent instruction of my German and Dutch coaches!)
The judges who understand what’s going on will make a comment that I have a very good, quietly effective riding position for my body type and those judges will score me at a 8-9, even got a 9.5 once. These are usually the tests that result in my winning the class or placing second, as long as other mistakes weren’t made.
I’ve learned that I have no choice but to make up for my physical pitfalls in other ways - I ride my horses powerfully through my seat and legs at all times, I am very accurate in my tests, I’m insistently anal about a horse learning and doing something right (whether at show or at home - I will sacrifice a test and TRAIN the horse through the test, so that at the next show he/she does the movement correctly), and I set strong but realistic goals for both horse and myself which we work very hard to achieve. My horses expect to work hard and expect me to work hard. Nothing less than that is acceptable for any of us. I reward the horse immediately when they do something right (especially if they do it right during a test at a show - I’m well known for leaning forward and giving a sound, one-time PAT on the neck for whatever movement they did right, especially if the horse has only just recently learned said movement, and then just carrying on with the test like nothing happened), and never let a horse get discouraged. [I also have been known to lean forward and smack a horse sharply for being naughty during a test - snap out of it already!!! - and heard more than one judge chuckle at that too!] And I laugh when a bad day just goes stupid-silly. When the stars align and all goes right, then wonderful. But sometimes you just gotta roll with the punches.