Every time I go to the craft supply stores I see fancy tools that help you make pompoms. I say, don't waste your money on those gizmos. I learned this technique when I was at the elementary schools and you don't need any special tools to make any size pompoms.
What you will need:
sturdy paper (I used a card that came in the mail with some ads), desired color yarn, sharp scissors, compass (or anything in circle shape - I used a glass that I traced around).
Technique:
Trace and cut two identical doughnut shapes and lay them on top of each other. The width of the doughnut will be the radius of the pompom. Wrap the yarn tightly around the width of the doughnut. The more yarn you add, the fuller the pompom will become. Find a spot to separate the yarn and insert scissors between the two layers of paper. Cut the yarn all around. Insert a yarn string where your scissors were and tie a knot. Make sure your knot is tight, otherwise the pompom will loose its fullness quickly and the yarn will fall out. Remove/tear out the two paper doughnuts. Use scissors and cut any strands that stick out. And there it is, a cute pompom to top hats, beanies, gloves, booties, etc. No special tools required.