Oh to be young again. As a kid I used to ride all sizes and never needed a mounting block etc - somehow you just scramble up there. When I was 12 (about 5'?) I was riding a 16.2 regularly, and used to mount no problem.
Anyway, with handling from the ground, size really makes little difference. Even a small pony is stronger than an adult, so relative size doesn't help you if your horse is bad-mannered. A well-trained big'un is better than a small naughty one.