Your ski sock size should match your regular shoe size — don't size up thinking bigger means warmer. A sock that's too large will bunch up inside your ski boot, creating painful pressure points and wrinkles. One that's too small pulls tight and restricts circulation, which actually makes your feet colder.
If you fall between sizes on a brand's chart, size down rather than up. A slightly snugger fit stays smooth against your foot and works much better inside a snug ski boot. Also, remember that women's sizing typically runs 1.5 to 2 sizes smaller than men's for the same foot length, so double-check you're looking at the right chart.
One last tip: size ranges vary a lot between manufacturers, so always check the specific brand's size chart rather than assuming your usual size will translate directly.