Dynamic light scattering (DLS) is a versatile tool for sizing nanoparticles (NPs) suspended in fluids. In this paper, we apply and discuss modulated 3D cross-correlation DLS and UV-vis-NIR transmittance spectroscopy for the study of commercial iron oxide NP aqueous suspensions (Fe3O4 and Fe2O3, with nominal average size 10 nm and < 30 nm, respectively) motivated by the aim to use them for the formulation of magnetic NP inks. By analyzing the particle size distribution with the LS Instruments’ CONTIN algorithm, we find that the predominant particle sizes in nanoparticle products can strikingly differ from the values that a user would typically expect according to the provided suppliers’ information. We thus highlight that for application fields of magnetic NPs, such as e.g. ink formulations for nanoelectronics, spintronics or printed electronics, and whenever the particle size matters, the purchase and use of NP materials needs detailed product information and investigation of particle size distributions. Our analysis as a function of NP concentration confirm that modulated 3D cross-correlation DLS can efficiently be used even at high NP concentrations (e.g. 1.17 g/mL Fe2O3 load) without further dilution.