Hey! I got the schematic to pop in the second time I clicked it.
I think it's ok to post here since it's not actually the pattern.
I left the pattern set on 'Misses size medium' and clicked 'calculate' and that popped up some numbers. You start knitting flat from one shoulder edge (right) across the middle of your back (placing the first marker at the center of your back = half shoulder width) (step 2) working toward one hand (left). Half shoulder width looks to be the distance from the center of your back to your side/arm pit/shoulder.
Then when you've reached the measurement for another half shoulder width (or 2 X 'half shoulder width' from the cast on), you place another marker (step 4 & 5).
Then you start working the left sleeve in the round - so yes, the sleeve width is the circumference of the sleeve at the shoulder. The cuff width would then be the circumference at where ever you decide to end the sleeve (mid-arm or wrist).
Does this help?