If you use thinner wool your FO will turn out smaller, thinner and drape"ier."
If you want to use this wool, you need to either
1) use smaller needles (appropriate for the wool), and make a bigger size.
2) use the needle size called for in the pattern, but cast on more stitches.
Give us the gauge for your wool and the gauge for your pattern, and we can be more specific for you.