The pattern includes instructions for making this with Álafoss Lopi (as shown in the photo included) and also with triple Plötulopi .
The instructions and needle sizes are the same for both options, but the 3x plötulopi will be a tiny bit bulkier. I recommend using Álafoss lopi for the sake of simplicity.
If you knit very tightly, I recommend going up half a needle size, using 6mm and 7mm instead of 5.5mm and 6.5mm