As for myself, in my younger days, I had a taller horse, and had no trouble mounting, but by the time I was 30, the horse I had was only 14HH, and I was glad she wasn't any taller. On some of the trail rides we went on, I got on and off so much that my left leg would cramp up. My mare was a stubborn witch who refused to cross water or bridges, or go through an underpass. After a few months of mounting and dismounting, she gave up and decided to cooperate.
My current mare is 15HH. I haven't tried to get on her, because I tried to get on hubby's 14HH gelding and it was IMPOSSIBLE. Then again, I'm nearly 53 years old now and bigger around than I am tall. I did manage to mount using a mounting block, but it was not a pretty sight.
I want to start trail riding again come spring. I hope the rides are short enough that I don't have to worry about mounting or dismounting while on the trail, or I may have a long walk back to the starting point!