These were a huge hit at the Art Clay World Conference. There are 4 to a pack and each ball is approximately 1" or 25mm in diameter.  You can use these for metal clay or polymer clay.  Simply place a thin layer of clay over the cotton ball.  When you complete your project you will have a hollow bead.  With metal clay the cotton will burn out when fired.  If covered with Polymer Clay you will remove the cotton with tweezers after you bake your clay.