I'm 5'3" and my horse is almost 16h and still growing. He's got at least a couple inches to go yet.
I'm not a skinny minnie anymore, and my knees have never been great, so I use a step up of some sort 99% of the time. Whether it's a stool, a stump, a hill....anything to help. It's much easier on my knees and also on my horse's back. I can get on him from the ground in an emergency, but there are very few places I ride where I can't use the terrain or an object to my advantage when mounting.