I knit tight so I used needels sized 4mm for all the hat.
If you knit more loosely, I recommend to use 3.5 mm needles for the rib

If you need to make the hat bigger or smaller:

Rememeber that your total amount of stitches in the ribbing should be devidable by 2, and after the ribbing your total amount of stitches should be dividable by 6 (because of the drecreases at the end). Increase you amount of sticthes acording to this.

Happy knit!