The size chart measures the garment, not you
A letter on a label is not a measurement. There is no standard behind S, M or L, no body that agrees on what they mean, and no obligation on anyone to be consistent. Two brands can print the same letter on garments four centimetres apart in the chest and both are telling the truth, because neither said anything.
This is why buying clothes online is harder than it should be, and it is fixable with a tape measure.
Two different charts
Most size charts are body charts. They tell you the body the brand had in mind: chest 96 to 101cm, wear a medium. That is a recommendation, and it hides the thing you actually need, which is how much cloth there is. A body chart cannot tell you whether a garment is cut close or loose, because the ease has already been decided for you and not disclosed.
Our charts are garment charts. Every figure is the finished piece laid flat and measured: chest across, body length, sleeve. It says so under every chart on the site, in the line that reads "these are garment measurements, not body measurements". It is a less flattering way to publish a size chart, because it makes a boxy fit obviously boxy instead of describing it as relaxed.
A body chart tells you who the brand thinks you are. A garment chart tells you what you are being sent.
How to use one
Take a garment you already own and like the fit of. Lay it flat, do not stretch it, and measure straight across from armpit to armpit. Double that if you want the full circumference, though most charts including ours quote the flat measurement. Then measure from the highest point of the shoulder down to the hem.
Now compare those two numbers with the chart. Not your chest, the shirt's. You are looking for the size whose numbers are closest to the garment that already fits you the way you want. That single comparison is worth more than any amount of guessing at letters.
Where the chart puts you between two sizes, size up. That is the note printed under every one of our charts, and it is there because cotton relaxes with wear rather than growing, and a garment slightly too generous stays wearable in a way one slightly too tight does not.
Two sizing systems, and why
Most of the line runs S to 2XL. The winged tee runs 4 to 12, a numeric system, because it is cut on a different block from a different maker.
We would rather have one system, and having two is a fair thing to criticise. What we are not going to do is relabel a numeric garment with a letter it was not graded to, because that would make the label tidier and the fit less predictable. The chart is the answer either way: measure the garment you own, read the numbers, ignore the name of the size.
Every product page carries its chart under the measurements, alongside the composition and the care. If a figure you need is missing, it is missing because we have not measured it, not because we are rounding.