Spooky.. I think that I used a size 4.5 mm
I'll look on my converter.. to give you USA sizes.. (the needles that I use are MM-- so, I've lost my USA mental conversion of needle sizes)..
OK, 4.5 USA = 7 (a size 8 would be fine too)
I just make them a little thicker (use a little smaller needle than you'd use to make a garment out of the same yarn) than you might knit a garment out of the same yarn.. does that make sense? So, they are strudy, warm, and able to stretch more- if needed.