This paper presents a systematic study on the role of particle size in pure and doped nanocrystalline TiO2 photocatalysts, which was made possible by a versatile wet-chemical process capable of generating near-agglomeration-free TiO2 with well-controlled particle sizes and dopant dispersion. It is shown that particle size is a crucial factor in the dynamics of the electron/hole recombination process. For TiO2 particles with 6 or 11 nm diameter, Fe3+ dopants were added to inhibit the charge carrier surface recombination. The optimal Fe3+ dopant concentration for different particle sizes was identified, and this concentration was found to decrease with increasing particle size. To assist electron and hole separation in TiO2 with 21 nm diameter, Nb5+ dopants were introduced in combination with minor surface Pt dispersion. These carefully engineered nanocrystalline TiO2 catalysts showed higher reactivities than Degussa P25 TiO2 material in photocatalytic decomposition of chloroform.