I just reallly want to know!
I did a quick goole search using "free knitting patterns leg warmer" and came up with a list of free patterns. I haven't made any of them up, so can't attest to their simplicity or accuracy. But it's worth checking them out. I would certainly focus in on the ones using yarn and needles for at least 5 sts per inch or less.
If it were me, I'd just pick a pattern stitch and work out the gauge with the chosen yarn. Then measure the calf circumference and - using the "magic loop" method, I'd cast on the determined number of stitches and start knitting. Since you can try them on during construction, you can keep knitting till you get to the right length. I'd include ribbing in any "fancy" stitches to allow the width to stretch or not - ankle to calf.