A size letter means nothing across brands. What means something is the flat measurement: lay the shirt down, measure across the chest an inch below the armhole, double it.
Every product page here lists that number in centimetres and inches. Compare it against a shirt you already own and like, and you will get the right size more reliably than by guessing from a chart.
If you are between sizes, size up. Our cuts are boxy by design and the extra room reads as intentional.