The recommended needle size is 4mm (US 6). To make a more dense (and hopefully longer lasting) fabric, I usually drop a couple of needle sizes. So I’m going to knit a swatch (in the round) on 3.5mm (US 4) needles.
To keep things simple for the purposes of this tutorial, I’ll knit the sock in stocking stitch (stockinette).
If you’d like to try out two different needle sizes, you could knit the swatch on two circular needles - one half of the stitches on one needle (say 3.25mm (US 3)) and the other half on another (3.5mm (US 4)). That way you get two swatches for the price of one!