Size guide
Every garment has its own chart, measured off that garment in the shop.
Where the chart is
On each garment's page, under the sizes, it says “see the size guide”. That is where the measurements of that particular item are, in centimetres.
There is deliberately no universal table. An S from one maker is not an S from another, and the clothes here are from different brands. One shared table would contradict the numbers you have just read on the garment itself.
What the two columns mean
| Width | From one underarm seam to the other, with the garment laid flat. That is half the circumference. |
| Length | From the highest point of the shoulder straight down to the bottom hem. |
The surest method
Take a garment that fits you the way you like, lay it flat on a table and measure it the same way. Compare those two numbers with the chart. It lies less than measuring your body.
If you are between two sizes
Call one of the shops. The garment is in front of them and they can measure it again or say how it sits. And if you still get it wrong, a size exchange is free in the shop.