Most footwear in the archive is listed in EU sizes, the size the brands produce. As a working rule for men’s shoes: EU 42 = US 9 = UK 8; each full EU size up adds roughly half a US size.
Women’s and kids’ conversions differ, and individual models vary. The full conversion tables, including insole lengths in centimeters, are on the size guide page.
If you are between sizes or unsure about a specific model, ask through the contact page with the style code from the product page.