What length stirrups do i need?

I am about 5'7" and im looking to get some stirrup leathers for the first time, for my first horse. if it makes a difference at all im oalmost 15 and im like 105lbs, so can somebody help me? the stirrups i was looking at come in 48in 58in and 64in lengths, and i have no idea which i need because i take lessons at a local barn where all the tack is provided for me, and my instructer always adjusts my stirrups once im on the horse. thanks ahead of time<333