I purchased this hat for my 5month old son for our trip to the beach.  I purchased the royal blue size 0-6 months.  My son has a small head for his age (50th percentile) and this size fit perfect.  There is an elastic sizer in the back that allows it to tighten & expand.  The small strings in the front helped it stay on.  He didn't mind wearing it and it protected his head, ears and neck from the sun.  I would definitely recommend it for protecting against the sun!