Sizing engineered nanoparticles in simple, laboratory systems is now a robust field of science; however, application of available techniques to more complex, natural systems is hindered by numerous challenges including low nanoparticle number concentrations, polydispersity from aggregation and/or dissolution, and interference from other incidental particulates. A new emerging technique, single particle inductively coupled plasma-mass spectrometry (spICPMS), has the potential to address many of these analytical challenges when sizing inorganic nanoparticles in environmental matrices. However, to date, there is little beyond the initial feasibility studies that investigates the performance characteristics and validation of spICPMS as a nanoparticle sizing technique. This study compares sizing of four silver nanoparticle dispersions (nominal diameters of 40, 60, 80, and 100 nm) by spICPMS to four established sizing techniques: dynamic light scattering, differential centrifugal sedimentation, nanoparticle tracking analysis, and TEM. Results show that spICPMS is able to size silver nanoparticles, across different sizes and particle number concentrations, with accuracy similar to the other commercially available techniques. Furthermore, a novel approach to evaluating particle coincidence is presented. In addition, spICPMS size measurements were successfully performed on nanoparticles suspended in algal growth media at low concentrations. Overall, while further development of the technique is needed, spICPMS yields important advantages over other techniques when sizing nanoparticles in environmentally relevant media.