If you can knit loosely across fair isle color changes then size 6mm needles will be all you need. I used 6.5 mm (US 10.5) needles for the fair isle segments to ensure it didn’t tighten up too much in felting in those areas.

There is a typo in the pattern that says 6.5/US11, it should be 6.5mm/US 10 1/2 needles, but if you use 8mm/US11’s that will also be fine, it’s just to have slightly looser stitches across any fair isle areas.