I have used Merino Yarn from Stoff og stil in the pink Hat. I used about 3 skeins.

The Blue hat is knitted with Bingo yarn, Lana Grossa.
I used 3,5 skeins of this yarn.

Using Big Merino, by Drops, you will probably use 3-3,5 skeins

I you want to nit this to an child, you simply cast om fever sts, and the hat will be less baggy.

You knit for as long as you need for the hat to fit your head perfectly :-)