medium size people on horses

I would class myself as medium sized, I take clothes size 12-14 and I'm about 5 ft 4-5... I would say that was bang in the middle for a lady?. My age (early 30s) is middling too. My horse is also middle-sized, being 15.2.

Is it OK to be a medium-sized person on a medium-sized horse? Is that the optimum combination?
 
Top