The best way to measure your correct size is to compare it to a similar garment you already own.
All measurements in centimeter.

Size	Waist	Length
S	21 - 26 cm	30 cm
M	21 - 28 cm	31 cm
L	21 - 30 cm	33 cm
XL	24 - 32 cm	35 cm
2XL	24 - 34 cm	37 cm