An existing model that relates the annihilation lifetime of positronium trapped in subnanometer pores to the average size of the pores is extended to account for positronium in any size pore and at any temperature. This extension enables the use of positronium annihilation lifetime spectroscopy in characterizing nanoporous and mesoporous materials, in particular thin insulating films where the introduction of porosity is crucial to achieving a low dielectric constant, K. Detailed results of the model calculations are presented along with extensive experimental results to systematically check the lifetime vs pore size calibration in a variety of low-K materials over a wide range of pore sizes.