She looks like she's 13 h or so. Hard to tell in the pic. I'm 5 nothing standing up straight, so there wouldn't be a problem.
She's strongly built too... She could carry an adult for purposes of training I think... Even if they'd look silly. I've thought many times of sliding up on her myself.
What's too bad is that you aren't closer and wanting to risk yourself for such a project. If she were well broke she'd bring a lot of money in pony club circles. Have you seen lately what those people will pay for jumping ponies? She's definately pretty enough, and athletic enough.