As far as your size goes, I ride a 14.1 (stretching) QH mare, about 850-900lbs and I weigh 160 and am 5'7, so unless it really bothers you, size is not an issue. Well, not for me and my mare, as as far as the trphy's show, we can still work some cows......but of course, she's bred to be that small.
However,I do think your mare will grow. My gelding, at 2 1/2, didn't grow for at least 6 months, and right at his 3 year old year, or a little after, he went from 13.3 to 14.3, so don't worry, I think she's still got some growing to do, and most horses widen out with age.