<p>Micro- and nanoplastics are environmental contaminants of increasing concern due to their persistence, ubiquity, and potential impacts on ecosystems and human health. Validating emerging detection technologies for these substances and enabling interlaboratory comparability requires the development of appropriate reference materials (RMs). Ideally, these materials should mimic plastics from the environment which are heterogeneous in size, shape, and morphology. Therefore, unlike more traditional RMs for chemical metrology, nanoscale plastic RMs must be characterized not only by chemical composition but also by particle size and mass-related metrics which presents new challenges in material characterization. To highlight these challenges, we fabricated nanoscale plastic materials intended for use as RMs from four different plastic sources. These materials were characterized using a variety of techniques, including light scattering and atomic force microscopy, to assess particle size distributions. We find that no single technique is sufficient to fully characterize the particle size distributions of these heterogeneous materials across relevant length scales. Similarly, determining the concentration of these nanoscale plastics in suspension remains nontrivial, with results depending strongly on the measurement approach and underlying assumptions. Our results demonstrate that reliance on a single characterization method can lead to incomplete or biased representations of nanoscale plastic materials. Instead, a multimodal characterization approach offers a more robust and defensible framework for evaluating size and mass characteristics of environmentally relevant plastics. This work highlights key considerations for the characterization and future development of nanoscale plastic materials.</p>