if it is stocking knit it will curl.
if it is supposed to have a curled (rolled edge/hem) you can do 1 row of K1, P1, (and the edge/hem will roll to this point(row) and no further.
you can block the sweater (and if its a small sweater, the blocking + the side seams will help reduce the roll--if it is an Extra large the side seams will help reduce the roll, but it will roll center front/center back (and no amount of blocking will resolve.)
Currently, there are many sweaters "designed" with no edges/hems or other features to prevent stocking knit from acting as it naturally wants to--(and rolling!)
if you really hate the roll, and don't want to fight it, think about adding:
1 a hem
2 an edging (knit on--see any of Nicky Epstein's books on edging, or any other book of the same sort, or even a good stitch dictionary for edging.
3--a crochet edging. (but a single row of single crochet will not do it.)