Round and cylindrical beads commonly come in sizes ranging from two to 20 millimeters in diameter. Square and oval cut beads have similar size ranges in two dimensions. Charts for basic sizes are available at Berger Beads, Big Bead, Little Bead, and Beads Galore International.
Berger Beads includes several different sizing charts for beads on its website, along with a wire gauge conversion chart and an aught chart for seed beads. Big Bead, Little Bead's chart of common sizes is available as a printable PDF. The website also offers a bead calculator for projects by length. Beads Galore's sizing chart includes quarter and credit card images for comparison purposes.