This pattern is basic knitting in the round with incremental stitch reductions for shaping. Sizing seems to be managed by using different weights of yarn rather than stitch count. The pattern lists the finished shoulder measurement as 40” for size small and 46” for the medium, but doesn’t give a whole lot of information about yarn, gauge, or circumference at the bottom edge.

My best suggestion is to make a swatch to determine how many stitches you need to get the measurement you want and go from there. In my case, my double strands of yarn had a gauge of 2.5sts per inch so I had to cast on substantially fewer stitches. (140 instead of 200)