Size guidelines

Gag size	The depth of penetration of the inner part	Your body height
XS	58 mm	< 160 cm
S	61 mm	160 - 168 cm
M	64 mm	168 - 178 cm
L	67 mm	178 - 190 cm
XL	70 mm	> 190 cm

Note: if your size is somewhere in the middle, or you are a beginner, then take the smaller gag. If you are more experienced user, then take the larger one.