Love my Buff- just wore it for the first time commuting today!
My head size is right around yours, Miranda. I went with the Women's Slim-Fit Buff (
http://www.buyabuff.com/Women%20Slim%20Fit/women.htm), which is really just a Junior Buff in more grown-up, feminine patterns. It fits perfectly! Buyabuff.com was the only site that had them- the Buff company site didn't even have them yet! Peter at Buyabuff/Green Extreme Outdoors Ltd. told me that they're a new product this year. He was very helpful and pleasant, and my stuff got here super fast from BC.
I tried DH's adult buff, and it was too loose, depending on how I wore it. My chick buff was a little snug at first, but has loosened up nicely.