How to measure headwear size?
Measure point of wearing headwear or thickest part of head.
(That length is usual headwear size indication,
if there's no specific instruction.)
If you can use measuring tape(soft), measure it directly.
You can use some string or thread instead.
Don't use steel material measuring tape.
After measuring, you can use any kind of ruler.
We are using centimeter unit.
If you only have inch unit ruler,
measured inch * 2.54cm

And choose bigger than measured size.