Its depend on preparation method. if it was wet chemical you can control by growth time, the concentration of the precursor such as TTIP and the temperature.
of course the method of preparation is important, in the the case of hydrothermal method the annealing temperature , concentration of precursor, the solvent and PH all can be effective. the following article may can be useful:
Article Synthesis of stable TiO2 nanotubes: Effect of hydrothermal t...