The annihilation characteristics of a positron trapped in spherical clusters of 1 . . . 50 vacancies in Al and Mo are calculated. The results are found to depend sensitively on the void size up to a radius of about 5 A, beyond which the size dependence disappears. The residual resistivity is shown to be nearly independent of the microvoid size during initial stages of vacancy clustering. The sizes of the microvoids in electron irradiated Mo as a function of annealing temperatures are evaluated from recent experimental positron lifetime results.