Swanndri - Shirts Relaxed Fit.
Arm pit to Arm pit measurement in centimeters | Measurements are accurate to within 1 - 2 centimeters*
The easiest way is to take an existing shirt that fits you well, lay it flat and measure across the chest from armpit to armpit.
This give you the half chest measurement once you have this, check the size chart and order the size closest to what you measured on your existing shirt.