Ceramic beads 18 mm are, next to 12 mm, the most frequently bought size, even called "classic eighteen" by customers. Good size for a massive bracelet, large but not too large earring and standard necklaces. Are you designing a necklace? Go from this size, add two smaller and two larger sizes, compose the colors and it's ready. If you want your necklace to have this "something" to the classic arrangement of balls, add asymmetry in color, or replace one ceramic bead with an openwork metal ball.