You will have to do some math, but it might not be that hard. You can likely do one of the smaller sizes with worsted and the needles of your choice. First, figure out how many sts/inch you get with them. Then how many inches around you need (36, 40...) and multiply your sts/inch by that. The number you come up with will be how many sts in the front and back combined at the largest part. Look to see which size has a CO closest to half the total sts needes and follow the directions for that size. Most of the instructions for length are in inches, you'll need to adjust that for your true size rather than the smaller size. On the waist shaping, their 5 rows is about 3/4" so do as many rows as you need that equal that measure. Same for the sleeves and the bust shaping, you'll have to translate their rows into inches, then inches into your row gauge. It really isn't too awful to modify it.