Ah don't worry about it at all :) Barn owner rides my horse sometimes and Cheyenne's 14hh the BO is about...5'7" and she looks to be about the same weight as you if not heavier. She also used to have a 11.2hh Welsh pony that she rode too. Horses are strong, you'd be fine on her :)
I would say it would depend on what you want to do. This horse is a great size for trails, working on the flat, or maybe small jumps. I am not sure that she is going over any 4 foot jumps with you. For my riding, she would be fine.