I would go up a size in the sleeves if there was more than one size, but the pattern only gives directions for one sleeve that is supposed to fit all 5 sizes. The fabric is quite stretchy, but the sleeves loose some of their puffiness as they're stretched. A larger sleeve cap would not be a problem, I don't think, as it's meant to be gathered at the top anyway. I suppose I am really looking for information on how to grade sleeves.
The problem with the difference between the shaping in the armhole and on the sleeves, is that the underside of the sleeve increases by 1 st on each side every row and finishes in a point under the arm, whereas the body has a 6 st bind off edge under the arm, and then decreases in steps of 3 sts on either side every other row. So it's a matter of joining something pointy to something flat, which I am having a very hard time visualizing. (I attached a pic of the underside of the sleeve, fyi one pattern column is 4 sts wide.)