Measure the head size just above the eyebrows and the ears.
Use a soft meter.

note * : You can measure 2 or 3 times and keep the mean value.

For the standard sizes, choose according to your head size:
Size L = head size 58,5 - 60/61* cm (* - measure dependent)
Size M = head size 55,5 - 58 cm
Size S = head size 52,5 - 55 cm

For BiOS FiX choose FiX in the BiOS SIZE field of the form
and specify your head size (cm) in the HEAD SIZE field.