Just look at the gauge. If you want fingering yarn, and you knit say 8 stitches to the inch, and the worsted directions are for 4 stitches to the inch, you will have to double your stitches you cast on. Of course, you may need to do more math than that as the numbers may not work out so conveniently. But that's it. Same for the number of rows. You will have to see what your gauge is and what the pattern's gauge is and do the math for the rows.

thanks Sheila, but after much searching I finally found a site (knittingfool.com) that does the calculations for you. Ellie