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.
Tissue Paper Pom-Poms - Armed with fluffy pom-poms, kids can tell parents what to do: Stand up! Make some noise! Construct these crowd-pleasers by cutting strips from layers of tissue paper and then wrapping them around dowels.