I def. agree it depends on the build of the horse. I rode ponies pretty much my whole life until I got my 15 hand Mustang. My Paint is 15.1 and he seems tall to me..lol but I am short. I often have the problem of my short legs cueing the horse correctly.
Mustangs are pretty sturdy horses, depending on what kind of riding you plan to do I wouldn't think you would have a problem. It is kinda hard to say though without us knowing you or the horse you plan to buy. I agree with maura about riding her and feeling comfortable as well as the pony being balanced and willing to move forward.
BTW: mliponoga, the horse in the pic that you are riding is a beautiful color : )
Horses and children, I often think, have a lot of the good sense there is in the world.
Josephine Demott Robinson Feed, muck, groom, ride. Repeat daily!