What size feather bonnet should I get? That’s likely your next question. Glens are sold in US hat sizes, so if you know your US hat size, you can order that.
How to measure for a feather bonnet: Ideally, you should use a soft tape measure to find the “circumference” of your head. Measure with the tape about an inch above your ears. You want the tape to be as tight as you’d like the hat to fit–measuring with the tape too loose or too tight can result in ordering a size that’s too loose or too tight. In a pinch, you could also wrap a piece of string around your head and then measure that length on a tape measure. In either case it’s best to have help, so you can get an accurate measurement.