Iron sucrose (IS), a nanocolloidal solution used in the treatment of iron deficiency anemia, is currently under investigation for the elucidation of its critical quality attributes. Assessment of IS's size and size distribution has been recently attempted using dynamic light scattering (DLS). However, due to heterogeneous interpretation of DLS data, variable results were retrieved. The aim of this work was to establish a simple and reproducible DLS protocol to unequivocally define the size and size distribution of IS by using size distribution approximation in Number. Underlining the limitations of the commonly used DLS approximations, we identified the drug as being composed of a population of monodisperse nanoparticles of about 7 nm in diameter. The method here described might therefore be useful for the evaluation of quality, safety and efficacy of IS and its follow-on versions.