The technique of using shadow images of particles, obtained in coherent illumination to measure particle size, is analyzed. The theory of Fresnel diffraction for an opaque disc was used to analyze shadow images of transparent spherical particles. A comparison of theoretical calculations with laboratory results supports the application of this approach to optical array probes. It is shown that the shadow image size of spherical particles is essentially dependent on the distance from the object plane. In particular, for drops with diameters of less than 100 µm, the errors in size measurement from the PMS OAP-200X may reach 65%. These results agree well with laboratory calibrations that use monodisperse water droplets. On the basis of calculated particle diffraction images, the shape of the sample area and its dependence on drop size were calculated. It was found that the sample area has a complicated sawtooth shape. Gaps oriented perpendicular to the axis of a laser beam occur in the sample area in the case of large droplets. Comparison of the sample area quoted by the manufacturer to the calculated one is considered.