To determine the best cap size for you measure your head as shown above, ideally using a flexible tape measure.
Measure the circumference of your head just above the top of your ears. Wrap the tape gently around your head making certain the tape lies midway on your occipital bone (that little bump in the middle of the back of your head). Told you, everybody has one.
Remember don't pull the tape too tightly and if in doubt you may want to order the next biggest size for comfort.