Submicrometer particles (polystyrene latices, for example) are often used as size standards for the calibration of various instruments and substrates. Ideally, the size distribution of such standards should be extremely narrow. However, the current method of choice for such classification, transmission electron microscopy (TEM), is fraught with problems rendering the reported size distributions uncertain. A far more precise and practical method consists of fractionating the particle sample prior to measurement by light scattering. A nominal 100 nm diameter NIST sample has been so-analyzed and yields a full width at half maximum of less than 0.5 nm, far below the resolution of TEM. The fractionation/light scattering protocol for so narrow a standard is confirmed by repeating the measurement using a broader sample fractionated under identical conditions.