I used 3 balls of violet Cotton Ease on size 6 Knitpicks needles.
I first started knitting this with the yarn doubled but I didn't like how it knitted up (too stiff and heavy). So I started again and using only one strand of yarn. It knitted up much softer.
I had read that many people find the pattern to be knitting up big. So if I was knitting it double I would do the size S. So I picked the size M to knit with one strand. This seems to have worked for me and I am very happy with the sweater.