Most T Shirt sizing are based on a half chest, or width measurement, see example below

We all have a t shirt we know fits us well, so why not use that as your guide.
To ensure you have the correct size for you, get your own shirt, lay if flat on a table and measure across the chest just under the arm pits.
This will give you your size in half chest; width measurement as above
These sizes are listed on the product page

Sizing(cm)	S	M	L	XL	2XL	3XL	4XL	5XL
Width	48	51	56	61	66	71	76	81
Length	71	74	76	79	81	84	86	89