You take a tape measure and put it around the head one centimer above the ears.
Hold the tape firmly, but not too tight. The centimeters equals your size.
(most common sizes are 57 to 61)
Let it dry naturally, not too close to a source of heat.
The caps are made of natural fibres. When they become wet, they might reduce slightly.
The easiest way to reshape the cap is to hold it above a kettle of steaming water and then softly reshape it on your knee.