The size of a glass bead can affect retroreflectivity, especially under wet conditions. Larger beads (Type III) have slightly higher retroreflectivity than standard beads (Type II) under dry conditions. Bead size is influenced by the size of the raw materials sent into the furnace. Note that bead size has no effect on refractive index. Bead size is determined in the lab using .

When markings are wet, beads are often rendered useless because the film of water that covers the marking causes light to scatter before it can enter the bead. This causes the wet markings to be nearly invisible at night. Large beads may be more effective when roads are slightly wet because their higher profile protrudes through the film of water better than small beads. Note that as the thickness of the water film increases, large beads begin to lose their effectiveness as well. Figure 2-18 shows light refracting through a dry glass bead versus the same glass bead with a film of water covering it.