Titanium dioxide is an effective photocatalyst when exposed to ultraviolet light. However, owing to its poor photocatalytic (PC) activity when exposed to visible light, its use as a photocatalyst has been limited. Recently, this limitation has been overcome by the use of mesoporous, anatase TiO2 microbeads codoped with transition metals and nitrogen, which greatly increases its visible-light PC activity. Further improvement in PC activity was achieved by creating codoped TiO2 microbeads of uniform size (monodisperse). The present study explored the effect of the size of the codoped, monodisperse microbeads on visible-light PC activity. Here we describe the results of experiments to find the optimum size of codoped, monodisperse, mesoporous TiO2 for PC activity.