This is a top down, traditional heel sock knit on double pointed needles.

U.S. 2 (2.75mm) for small/medium and U.S. 3 (3.00mm) for medium/large

If you wear a small/medium, but your leg circumference is greater than 12 inches, I would suggest you do the leg part using the 3.00mm needle. Then drop down to 2.75mm needle when beginning the heel flap.