To measure head circumference for a hat: wrap a soft tape measure around your head where the band of a ball cap would normally sit. Start at 0 (zero) on tape measure at mid forehead, wrap around above one ear and at the base of the neck, to other ear and back around to meet up with the 0 at start. 

measure head for hat, handmade toques made in Canada
If you do not have a flexible tape measure you can measure the same way with a piece of string or yarn. Mark it and then stretch it out and measure
with a meter stick or retractable tape measure. 
O/S (one size) beanies typically fit adults with a head circumference of 21 1/4"- 22 1/2". If you need a custom size please see product page for information.