Measuring Your Head Size
To find your head size, wrap a tape measure around your head, just above your eyebrows and over the widest part of your head at the back. The tape measure should remain basically parallel to the floor. Put your finger under the tape measure so there will be a little bit of ease. Check the measurement, and that's your head size.
It's best if you have someone do the measuring for you, but you can do it yourself in front of a mirror. If you don't have a tape measure, you can use a ribbon or a piece of string, or anything, really, just as long as it's not stretchy. Mark the spot on the ribbon/string, then measure that with a ruler.
Remember the old adage: Measure twice, cut once.