Here's a quick review of seed bead sizing for my new beaders out there: Seed bead sizes decrease as the measurement number increases. In other words, a 15/0 bead is smaller than an 11/0 bead, which is in turn smaller than an 8/0 bead.
In general, round (or rocaille) seed beads come in sizes 15/0(or 14/0), 13/0, 11/0, 8/0 and 6/0.
Japanese cylinder beads (Delicas) are available in size 15/0 (small), 10/0 (medium), 11/0 and 8/0 (large).
Heavy Metal seed beads area available in 15/0, 11/0, 8/0 and 6/0.
One version of seed bead sizing says that the size (11/0) is the number of seed beads in an inch. Supposedly, if you line up eight 8/0 beads, eleven 11/0 beads and fifteen 15/0 beads they should all measure one inch. Others say that the size approximately indicates the number of beads that you can line up in a 20mm length. Get a metric ruler (the one they gave you for free at the last big bead show) and line up eleven 11/o beads, and they should measure around 20mm. A third source says that the seed bead size is related to the size of glass rod used to make the beads. It's hard to say what it really means, but the important thing to remember is that bead sizes are in inverse proportion to the size of the bead. A size 15 is almost invisibly tiny, and size 5 is giant.
In reality, there is great variability in the actual size of beads, depending on the manufacturer or source of origin. The actual size of a bead differs greatly depending on finish and manufacturer. Japanese 11/0's, for example, tend to be a larger than Czech 11/0's, and the size of a bead with a lot of effects on it will be noticeably larger than a bead without effects. For example, the basic transparent 11/0 will be noticeably smaller than the metallic iris 11/0. They can be up to 20 percent larger or smaller.
Toho has an amazing shape chart on their website that is an incredible reference -- along with the amazing colors of all of their beads. It gives the actual size of their beads in millimeters. It really helps to see all the sizes and shapes together. Toho Beads
Miyuki also has a great chart: Miyuki Size and Shape of Seed Beads
Check out Matsuno's Size and Shape page
There is also a Preciosa Seed Bead Information Page
Here are some great references on this subject