Copyright © 2019 American Chemical Society. Size homogeneity is a critical factor in defining the effective room temperature magnetism of magnetic nanostructures yet a crucial issue toward their implementation in biomedics. Using field-modulated dynamic magnetometry, we studied characteristic size distributions of magnetic nanostructures empirically extracted from their spectral magnetization responses for a given frequency band. This spectroscopic method allows us to estimate magnetodynamic moments oscillating within their distributive activation volumes in addition to the relaxation times. We practically extend this approach to evaluate colloidal dispersity statistics of magnetic nanorods from the primary-to-secondary volume ratio.