Measure the circumference and/or width of your hand at the knuckles.
The circumference will give better results, but the width is easier if you don't have a tape measure.

 

Size	Circumference	Width
XXS	18.5 cm	8.0 cm
XS	19.4 cm	8.3 cm
S	20.4 cm	8.7 cm
M	21.3 cm	9.1 cm
L	22.2 cm	9.5 cm
XL	23.1 cm	10.0 cm
XXL	24.0 cm	10.5 cm