A system and method for measuring small particles suspended in a fluid are disclosed. The system employs optical imaging using diffraction enlargement. A sample of small particles illuminated by a light source is imaged onto a pixel array of detector elements using an imaging optical system having a reduced magnification not sufficient for forming a large enough image of a smallest particle of interest. A low-aperture imaging optics with NA<0.05 is used to add diffraction enlargement to the image corresponding to at least 5 pixels to enable accurate measurement of images of smallest particles of interest, and to increase an optical sampling volume. Suitably programmed processor is used for determining at least a pixel count for each of the diffraction-enlarged images, and for generating a number, size or distribution of particles accounting for pre-determined diffraction enlargement of particle images of different sizes. The method enables analysis of large samples of small particles in one measurement.