I would do a 6 inch or 8 inch length for that age. If you say she is petite, 6 inch might work better. I call those the cheeky tutus, cos it is short enough to see just the bottom of their diaper covers.
For a 6 inch length, you will need to cut the tulle 14 inches long. I am assuming you are doing the no-sew kind right? If you are sewing, then just 12 inches would be enough.