This pattern will get you a fingerless mitt approximately the size of a ladies medium. It should be easy enough to modify to different sizes by simply increasing/decreasing the number of rows the cuff is worked to, and possibly by adding/subtracting a few rounds in the hand. If you want to try to modify for a different size, please note that the cuff should just fit around your wrist without stretching.
Also, if you like a narrower cuff, begin with fewer chains. Keep an eye on the instructions, as you will change from working in joined rounds to working in the spiral
Items needed: One skein Patons Classic Wool Merino (or favorite light worsted yarn) F hook, Yarn needle