Great tutorial for how to make tissue paper pom poms also includes a Tissue Paper Pom Pom Size Chart. It tells you how many sheets to use for each size pom pom as well as what size the tissue paper sheets should be.
Tutorial: DIY Tissue Paper Pom-Poms this tutorial is pretty good. Use square sheets or tissue for a rounder Pom Pom. Tie the middle with ribbon and tied several on ribbon and hung them for a backdrop. Use thin tissue paper and fluff gently.