If there's a 50" chest, use half the sts and rows on that size. There's basically no shaping for the front and back, they're squares. Then you can seam them together and pick up sts around the armholes for the sleeves and knit down, knit straight for a couple inches, then decreasing every inch or so.
It might work using sport weight yarn and a small needle, maybe a 4 or 5 and see what gauge you get with it. You may be able to follow the smallest size as far as stitch numbers go. If it's still a little big, size it down by using 4 or 5 less stitches per 'size'. The difference in the cast on numbers is that many between sizes.