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 Open leg
S 76 cm 113 cm 20 cm
M 84 cm 114 cm 21 cm
L 86 cm 114 cm 22 cm
XL 94 cm 115 cm 23 cm
2XL 98 cm 117 cm 24 cm