It really depends. A pony of her size will probably not go too high just because most horses will be above 16hh. It would be a good experience, though, if you wanted to do some lower level eventing. What breed is she, that will help determine the sturdiness of your pony.
I know a pony that size gearing up to move up to Prelim. Honestly, size doesn't matter. If your horse has the talent and the heart to get there then with some dedication and a good trainer you can get there.
Yeah size does NOT matter!!! Theodore O'Connor is like a pony legend. Around 14hh competed Advanced level eventing and rolex a few times. Ponys are AWESOME jumpers. I wish I was short so I could buy one, they're the best. Haha.
You'll be fine ;). I'm 5'1 and started getting my toes wet in eventing on my 13.1hh welshxmorgan pony. Sure, they'll never do the bigger stuff, but if they enjoy jumping and are bold you can still have a blast. In our first XC clinic the clinician loved my pony the most out of all the horses - she was the most bold, forward horse there.
This was us last fall (wet day with a muddy landing side + a forward pony = not as much release and a more tense jumping position, ugh)