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	52 cm
M	21 - 29 cm	55 cm
L	21 - 31 cm	60 cm
XL	21 - 33 cm	62 cm
2XL	21 - 35 cm	64 cm