First figure out what tension you get when you knit the thinner yarn, it maybe be 15 sts/10 cm. Then once you have your gauge, multiply that by the cm you need to fit you. See if one of the larger sizes has close to that number of sts in it and follow those instructions, adjusting for your length. If not, take the size you would knit, convert the sts into cm and translate that into the number of sts you need to get that measurement and use those numbers.
sue- knitting heretic