What you can do is knit a little swatch. See how many stitches per inch you get with your yarn and needle. Then mulitply that by how many inches around you want the leg warmer to be. Cast on that many stitches. You can do ribbing at the beginning and end to keep them on, maybe in a needle size a bit smaller than the main part.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry