You may have to go 2 or 3 sizes down. CO about 20 sts and knit a couple inches, see how many stitches per inch you get. Then divide that into the stitches given for the various sizes just below the underarm, and see which one is closest to the finished measurement you want. If you just start in on the next size down without check, it may turn out too large.
sue- knitting heretic