The TEM images of the samples were recorded on a Talos F200S Microscope (ThermoFisher Scientific) at 200 kV. The samples were prepared by drop casting 10 µL of a nanoparticle suspension onto carbon-coated copper grids and dried at room temperature. For gold nanoparticles the grids were subjected to plasma cleaning with a gas mixture of 25% Oxygen and 75% Argon for 8 seconds prior the drop casting to improve the wettability of the grids and achieve better dispersion of gold nanoparticles on the grids when drop-casting from aqueous suspensions. The obtained electron microscopy images were then analyzed by manually measuring the maximum Feret diameters of the particles from at least 3 different images with the software ImageJ. For the reference material candidates Au NP and SiO2 NP, automatic image segmentation with a convolutional neural network was performed for at least 1000 particles additionally, and the results are compared with manual measurements of at least 200 particles. For all other particles systems, at least 120 particles from at least 3 different images were measured manually.