I did use Drops Big Merino yarn for both my hats.

Fantastic yarn!!! Supersoft, and so warm :-)

You will probably use 3-4 skeins of yarn to make this hat. Depends of how tight you knit, and also head size :-)

I used 3 skeins, but then it is a bit tight for me. My head is 54cm.

The hat is knitted around the head, and makes it easy to fit perfectly!