One way to do this is to look at the difference in stitch numbers between sizes. So for instance, the CO numbers would be 8 sts difference, you would maybe want to go up 3 'sizes' from the largest given, so you would CO 24 more sts. It should work similarly with the arm shapings. Each size starts with a different stitch number and you do a different number of the decreases depending on the size. See if that would work out at your gauge so you'd end up with the right measurements.