It's very important to measure your head correctly.  There are a few things that you need.

1- A flexible measuring tape such as a dressmakers tape.  These can be found in any sewing/hobby shop, dollar stores, department stores and any other stores that sell a mending kit or simple sewing supplies.  Try your mending kit first if you're not a sewer

2- Your hair styled in a simliar fashion to how you would wear it when you wear your hat. ie: If you always wear a bun and you are looking for something to fit over it then measure with the bun in.

Technique:

 Hold the end of the tape against the middle of your forehead with one hand and with the finger under the tape.  This is important as it will give you a comfortable fit.   With the other hand, wrap the tape around your head just over the ears and overlap the first finger.  Do not pull tighlty.  It should be snug but not uncomfortable.  Hold the tape on the spot where it overlaps.  This number is you head measurement. 

My Small hats fit 21 1/2 " to 21 3/4" heads. Medium fits 22 1/2" to 22 3/4" heads and Large fits 23" to 23 1/2".  There is always a slight variance since they are all handmade.  Up to 1/8" can be added through gentle stretching and a little steam from the kettle.