Excerpts from the website

I used size 9 needle, but you may need to go down to a size 8. I tend to knit a little tighter than most knitters. Knit a swatch if you’re in doubt.

General hat tip: I have learned from knitting approximately one gagillion hats that for most adults you can estimate a good fitting hat at 18” of finished fabric. This particular hat is a bit slouchy, measuring in around 20” in finished diameter.