Yarn:
Chunky weight #5 yarn - Any yarn of this weight with a recommended hook of 6.5mm should work. I recommend Lion Brand Wool Ease Chunky (which is what I used in the photo, however, you will need a little more than one ball with that yarn. The color in the photo is Wheat) The other brand I recommend is Big Twist Chunky (you will only need one ball if you choose this one)

Sizing:
Because of the stretch of the brim of the hat, I would say it is one size fits most. However, I do have an entire page dedicated to resizing.