Outdoor Research Oasis Sun Hat (women's)

Lightweight, quick-dry, water-resistant hat with UPF 50+ sun protection. There is dark fabric under the brim to reduce glare, and a drawcord at the back of the head that can be adjusted with one hand. The floating brim is foam-stiffened and doesn't flip up in the wind as easily as other broad-brimmed hats do, and the chin strap keeps it on your head. This is not only a great sun hat, it shields you from light rain as well so you don't have to put your hood up.

To size, measure around your head where the hat would fit comfortably.

Small: 53.5-55 cm 
Medium: 55-56 cm 
Large: 56-57.5 cm 
XL: 57.5-59 cm