She's cute, agree that she is butt high right now, looks like she has growing to do yet, and now you have her you can get her measured so you know how tall she is. The string test will give you a good indication of growth potential, provided that she gets enough quality food to meet the needs of her and the foal while she is carrying, feeding.
I agree with Loosie, something a little funky looking with that back, but wait until she foals and settles down before you decide how that is going to be.
Saddle fit, should not be an issue, the UK is full of short backed, wide, cobs and native ponies, makes life easier.
She is a sturdy looking girl, and I don't see why she wont carry 5' 7" and 11 stone (154 pounds) when she is fit.
“Never attribute to malice that which can be attributed to stupidity”