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While mechanical behavior of crystalline materials has been well-understood in terms of lattice defects, metallic glasses pose significant challenges in defining the defects due to disordered structure. Physical/computational simulations have shown that equiaxed clusters, termed shear transformation zones (STZs), undergo shear transformations by atomic rearrangement in them to accommodate macroscopic strain. Properties of the STZs in amorphous Al86.8Ni3.7Y9.5 were characterized using their anelasticity as a probe. A combination of cantilever bending and bend-stress relaxation measurements was employed to measure quasi-static anelastic strain over ~1sec.–8×107sec. Direct spectrum analysis (DSA), the reliability of which was assessed with simulated data including noise, was performed to obtain the relaxation-time spectra. The spectra exhibit distinct peaks, which were analyzed in terms of the standard anelastic solid model with a linear combination of spring-dashpot, in conjunction with transition-state theory. The analysis provided the size of STZs: Active STZs consist of 14–21 atoms, resolved by a single atomic volume. The present model elucidates prior observations, (a) activation energy spectra obtained by the temperature-stepping method and (b) alpha and beta relaxations in loss moduli. The same STZ sizes were observed in both as-quenched and relaxed samples, suggesting that the activation energies are not altered by structural relaxation. The volume fraction of potential STZs (cn), however, decreases substantially due to relaxation. These are interpreted in terms of the free-volume model, modified to account for STZs as basic units. The resulting expression, fitted to cn, indicates that ~2% decrease in free-volume reduces cn substantially, leading to a viscosity rise. Published dynamic behavior of amorphous Zr46.8Ti8.2Cu7.5Ni10Be27.5 was analyzed in order to evaluate the model used. DSA, performed with loss modulus data, yielded distinct peaks in the relaxation-time spectra, consistently for all data sets. The analysis of these spectra, employing simultaneous fits to account for Arrhenius behavior of respective time constants and their size-dependence, yielded STZs sizes of 25–33 atomic volumes. Compatibility of the size, attempt frequency and transformation strain of an STZ below and above Tg is noted. The characterized STZ volumes are part of a wider hierarchy, and the window observed is determined by the experimental conditions.