The sizes correspond to the head measurement.
To measure it, place a measuring tape on the bump at the back of your head (usually 0,5 to 1 cm above your eyebrows and ears) and in the middle of your forehead (where you place the hat).
S = 55-56 cm
M = 57-58 cm
L = 59-60 cm
For a present, we recommand you chose M size (standard).