Our sizing is based around average, reasonably fit people. It should match the size you'd choose on the high street, not a cycling racing brand. Brands tend to have wildly varying size policies. Ours is "real".

UK Size 3-6 6-8 8-10 10-12
EU Size 34-38 38-42 42-44 44-48
US Size 4-7 7-9 9-11 11-13

Socks are very stretchy, so there's some wiggle room here.
Merino socks will seem to shrink when washed. Don't worry, they haven't. Just stretch back or iron.