Description from Knitty.com:

A note on sizing: arm length, arm width, and back width can all be adjusted independently. For wider arms, cast on more stitches (in a multiple of 12) and work more pattern repeats per round. For longer arms, do more pattern repeats in the elbow section. For a wider back, do more pattern repeats in the back section. If your back measurement lies between two sizes, work the back to the longer measurement and work the arms a bit short. You can then sew the center slit closed by a couple of inches on either side.