I believe one of the Stitch n Bitch books tells how to do it, you could check your local library for a copy.
What you would do, presuming you've got the same stitch gauge, is figure out how many inches you need it to be. Look at the pattern to see how many CO stitches between each size, then decide how many times that number would be needed to make it your size. Do that for back, front, sleeves. If the pattern has BOs for underarms, again, there's a different number of stitches for each size and you figure how many you need for your size.