Getting the size is easy. You need to measure your head in this way:

1. get a tape measure,or a string to be measured later with a rigid tape measure
2. pass it behind the head
3. place the beginnning of the tape measure, or an end of the string, in the center of the forehead
4. keep about 1 centimeter above the ear
5. join the ends of the tape measure/string, without tightening too much

The measurement, in centimeters, is the hat size, according to the European system (EU in the table below)
(To measure correctly, I recommend that you stand in front of a mirror)