Using a ruler, measure the length from the first joint of the middle finger to the wrist. Guidelines for size
Size S: approx. 13cm - 15cm
Size M: approx. 15cm - 17cm
Size L: approx. 17cm - 19cm
The above are guidelines. Depending on the length and thickness of your fingers or the width and thickness of your hands, the guidelines may not apply.
If the product does not fit your hand, use a cutter, etc. to cut away the part that does not fit.