Find out what size street shoes the girl wears. This is the best indication of what to look for in tap shoes.
Trace an outline of the girl's feet on a piece of paper then measure the distance from the heel to the end of the longest toe and the widest part of the foot. See reference for guide on how to convert these measurements.
Visit a store to try on tap shoes. As with all shoes, you will find variations in sizes between manufacturers. Keep in mind that tap shoe leather is less forgiving than soft-sided training shoes.
Select the smaller, if the girl's feet are in-between sizes. It is better for the tap shoes to be tighter than looser.
Wear socks when trying on the shoes, preferably the kind of socks you wear when dancing.