Use whatever yarn or hook size desired. The hat size will be dictated by the initial chained amount. For this particular version, I chained 75 with a sport weight and a Tunisian J hook. This fits an adult.
This pattern is extremely simple but allows you to try something possibly new - mattress stitch seaming! The first picture shows the main body of the hat. I purposely did not add a brim because I liked the slight curl of the knit stitch. But feel free to add any kind of brim you can think of!
The second hat shows the seam. Barely visible! And not bad for a hat not worked in the round. :)