The best way to measure your correct size is to compare it to a similar garment you already own. Lay that garment flat and take all measurements from the outside.
All measurements in centimeter.

Size	Chest	Length	Sleeve
S	51 cm	76 cm	65 cm
M	53 cm	77 cm	66 cm
L	55 cm	78 cm	66 cm
XL	59 cm	80 cm	67 cm
2XL	61 cm	80 cm	69 cm