To be honest, there's no way of telling. I bought a 5 year old 13.3 gelding. We measured him with a proper stick before we bought him. 18 months later when we sold him (sadly wasn't the right one for my daughter) he was 14.1. My other daughters gelding was 5 rising 6 and measured 14.2 with new shoes on. I can't get him to measure over 14.1 with new shoes on now (he is rising 9 currently), so try and explain that one!
I think there's plenty of growing time left for your girl...