You can knit with thinner yarn and smaller needles for one thing. But you need to knit a sample with those needles to determine that your sts per inch divided into the st number for one of the sizes will work for you.
You can also look at the number of sts difference in the sizes and go down one or two 'sizes' from the smallest. Say the CO numbers are 85, 92 and 99 for s, m, and L - that's 7 sts difference between the sizes, so to go down one size you would need 78 sts. Do that for every part a stitch number is given. You may also need to do less increases and decreases for a smaller size to make sure the length fits you.
sue- knitting heretic