If you want to cut both sleeves at the same time, place two pieces of fabric folded in half such that the fold is on the top edge. Use your sweater as a template to decide what size to cut the sleeves. Cut your sleeves a couple inches longer than you want the final length to be, so you have enough fabric for the hem, and leave a 1/2 inch seam allowance. If your fabric has a pattern or print (for example mine is ribbed), make sure that the direction of the pattern on the sleeves matches the direction of the pattern on the body when you are wearing the sweater and your arms are down.
I cut my sleeves 73cm long, and the wrist edge measured 12cm (so the diameter would be 24cm). The shoulder edge measured 27cm (diameter 54cm).