IMO, as a long time rider of smaller Arabians, if you're 5'2" and 115, you're not too big to ride anything but maybe a mini. If she's over 14.2 she's a horse not a pony, so I wouldn't worry about it. It depends on how comfortable YOU are on her. For the long, low, slow paced hunter classes I would prefer a larger horse because of the striding. For true Jumpers she might not have the scope to be able to pull off some of the larger jumps. But for your basic 3' to 3'6" jumps on a hunter course, I wouldn't worry about it for a minute.
And you're correct, at 19 you're no longer a junior rider. You're in the 18-35 age range at most shows now.