Kingsley is a great unisex hat pattern with sizing for either a more traditional fitted look, or a more relaxed slouchy fit.
Finished measurements are a 18" (20") circumference unstretched (which has enough stretch to fit a large range of sizes), and heights between 8.25" and 9.25".
If you want to get extra excited - this hat is just begging for a nice pom to top it off!
A note on yarn choice: If you want to get the most slouch bang for your buck, I highly recommend using a yarn that has some alpaca in it! I have found that alpaca really adds the best softness to your fabric to get that great slouchy drape. I personally prefer a yarn that is a merino/alpaca blend. The merino wool gives some nice elasticity (so that your hat doesn't lose all of its shape), alpaca gives you your slouchiliciousness (yes that's a word) and both are awesomely soft so you don't get the dreaded itchy hat head.