It's probably going to be more of a pain to try to take in the edges of it than it will be to start over. Since you're only partway done with just one piece, it's not that much of a hassle to reknit it. Even advanced knitters goof on sizing too and have found it's better to start over than jigger the pattern around, it just doesn't fit right.
Leave that piece alone for now, cast on for a sleeve and use it for your gauge swatch to check that you match the gauge, or are getting the finished size you want. Use the next size smaller and work about 6" of it. Then measure and see how it compares to the size you need. If it's okay, then go ahead and finish the sleeve and you can do the other pieces in the same size. If it's still not right, then we can help you some more.
sue- knitting heretic