SomethingGlassy.com, BeadsGalore.com and Beadaholique.com are three websites that provide bead sizing charts. They offer size conversions, visual aids and specific information about sizing seed beads.
SomethingGlassy.com provides a bead sizing chart and an inches-to-millimeters conversion table. It also offers approximations for how many beads fit on a piece of jewelry per inch. BeadsGalore.com gives a visual aid using a credit card and a quarter for size comparison as well as a millimeters-to-inches conversion table and formula for configuring an appropriate number of beads to use per strand. Beadaholique.com specifically provides information for sizing seed beads by bead type and includes recommendations for the type of thread to use with each type of bead.