Finished headband is 2.5 inches wide at the top and 18 inches around. To fit an average sized woman. But you can easily make the part that covers the back of the head longer or shorter for a custom fit.

This pattern makes a wide headband embellished with drop stitches, with a Garter Stitch band that goes around the back of the head.

This pattern was on About.com (which no longer exists) and has been deleted from their site, but for now it can still be found on the Internet Archive. They redirect and delete pages a lot so you may want to download/print to save.