  The present paper proposes a novel model for estimating the free-volume size
of porous materials based on the analysis of various experimental
ortho-positronium ($o$-Ps) lifetime data. The model is derived by combining the
semi-classical (SE) physics model, which works in the region of large pores
(pore size $R >$ 1 nm), with the conventional Tao-Eldrup (TE) model, which is
applicable only for the small-pore region ($R <$ 1 nm). Thus, the proposed
model, called the hybrid (HYB) model, is able to smoothly connect the $o$-Ps
lifetimes in the two regions of the pore. Moreover, by introducing the $o$-Ps
diffusion probability parameter ($D$), the HYB model has reproduced quite well
the experimental $o$-Ps lifetimes in the whole region of pore sizes. It is even
in a better agreement with the experimental data than the most up-to-date
rectangular TE (RTE) and Tokyo models. In particular, by adjusting the value of
$D$, the HYB model can also describe very well the two defined sets of
experimental $o$-Ps lifetimes in the pores with spherical and channel
geometries. The merit of the present model, in comparison with the previously
proposed ones, is that it is applicable for the pore size in the universal
range of $0.2 - 400$ nm for most of porous materials with different geometries.
