Positron annihilation lifetime spectroscopy is extensively used to get information on the sizes of the nanoholes existing in porous and nonmetallic solids. Cavities are most often modeled as spherical, although their shape is irregular. In the present paper the relationship between lifetime distribution as extracted from a time annihilation spectrum and size (and volume) distributions of the holes for some nonspherical geometries is discussed. It is shown that the same lifetime distribution can produce cavity size distributions having different momenta (centroid and standard deviation), if the shape of the hole is modified. Nevertheless, the ratio between the two momenta is largely independent of the geometry. Furthermore, parameters derived from the positron measurements (such as the free volume fraction) are not strongly influenced by the choice of the hole geometry.