Some of my friends are in the process of knitting hats for Naomi. I saw this graphic in a private group I'm in and got permission to share it here. It's not already saved anywhere public on the web, but the credit goes to Alie Jones who blogs at http://blog.doublejones.com/category/baby-amelia/
Click on the picture to see it full size. The text says "5 inches from base of neck to top of forehead" and "7 inches from ear to ear (over top of the head), 7 inches around the back of the head at base of neck."
A small string is definitely helpful to keep it tied on. Also some varying sizes are good, as we won't really know what will work best until we try them on her.