Choose some clay. The best clay to use is porcelain. I used a stoneware clay here for this tutorial because it photographs easily.  The reason porcelain is best is because it is smooth and after it is bisque-fired, it will be very porous.  Because it is porous, it will be easy to remove your clay ball.

Take some clay and make a ball.  If you want a big beads, use more clay.  I suggest making a lot of these and doing a bit of experimenting. At the very least, make five balls and have five sizes in mind: ex-small, small, medium, large, and ex-large. 

Push it flat with your hand and turn it over so the flat side is on top.

Make an indention.   If you want a precise measurements, say 10 mm, 12 mm, 14 mm, 16 mm and so on, my suggestion would to make about 20+ balls and start making different sized indentions.

Guestimate! Guestimate! Guestimate!  These will be your sizes of beads. Make as many of these as you feel like making.  Let them dry and bisque fire them to Cone 06.  These are your OWN personal Bead Measurers!

This is a porcelain bead measurer that I've already bisque-fired.  Using your thumb, push in some clay and scrape the rest off. This is why it is important to have a flat top. Use the same pressure with every bead.

Pull it out.  In order to pull it out, you'll have to really push in with your finger and then pull.  If your clay is too wet, forget it. If your clay is too dry, it cracks when you try to roll it.  But when the clay is moist and plastic, the beads are easy to pull out.

Roll your bead.  Make it any shape you wish.

After you have glaze-fired your bead(s), measure each with a pair of calipers.  Those will be your sizes of beads.  My sizes are 10 mm, 12 mm, 15 mm, 18 mm, and 20 mm.  If you want a precise measurements -- say 10 mm, 12 mm, 14 mm, 16 mm and so on -- my suggestion would to make about 20 balls and start making different sized indentions. Have fun!