I am in lurve with this slouchy hat pattern. My inspiration was my brilliant hair stylist who wanted a hat like this. She called it Reggae styled and I had no idea what she was talking about, but then I figured it out and it's great! After browsing the Internet and failed attempts at finding a pattern online, I found this book at Jo-Ann Fabric.
I usually make the small/medium sized hat. You will notice there is not much a difference between the small/medium pattern and the medium/large pattern except an extra row of single crochet at the end. I had to do some adjusting for a smaller head, but it's pretty easy to incorporate more aggressive decreasing in the pattern. I also had to change the hook size when using Red Heart soft yarn. Even though it's just a little thinner than Bernat's Super Value yarn, it makes a big difference when you are making the hat. My head is 23 inches around by my eyebrows and the small/medium fits perfectly. However, my head is really more of a medium/large. Please keep in mind my tension is pretty loose when I crochet, so if you crochet more tightly you may need to adjust in a different way.
My favorite thing about this pattern is that it's super easy to make, but looks amazing. It also incorporates a lot of crocheting in the back loops only which makes a fabulous textured look. The photos don't do it justice! You can also create endless combinations of stripe designs with two or more yarn colors. Make one for yourself and pretty soon you'll be in slouchy heaven.