I'm 5'3 and I've ridden 17hh Hanos, 12hh Donkeys and just about everything in between size wise.
Just remember that it's still a horse, but more of a cruise ship size than speed boat or small yacht sized. Stay on your toes and really ride proactively. Learn to anticipate the horses, and ride more with your seat and legs. You weight about as much as a horse's head, so don't ever get in a pulling match because you are going to lose. And have fun! Your trainer is always going to be there to help you out of sticky situations and teach you to ride a horse so just stay calm and really try to ride with the horse. If the horse did a huge lead change and you fell onto it's neck, first of all this tells you that you weren't riding proactively to get a smoother change and second, that your back and hips were too stiff to move with the horse's movement. Don't stop riding as soon as the horse size and stride length goes up.