Despite the variety of flight task preparation programs represented in the market and introduction of new standards of technical requirements in topographic aerial photography, the design issues of the latter are not worked out in detail; the criteria of pixel size selection on the ground are not defined. The necessity of considering all the input data i.e. customer requirements to the outcome product, characteristics of the used technical means of flight support, navigation and photographic equipment, the impact of external factors of the photographic environment was noted in the paper. Within the framework of the unified methodical approach to the choice of the pixel size in the design, the use of theoretical and physical similarity criteria is proposed. The method provides analyzing aerial photography’s all external and internal factors complex interaction and consideration of their affecting the result. Physical criteria, i.e. stability of flight, image quality of images, and depth of depicted high-altitude objects sharpness are considered in detail. The theoretical justification of determining the similarity coefficients is given. It analyzes the changes in coefficients depending on focal length, camera exposure parameters, etc. The method of taking into account physical factors and estimation of design accuracy is presented. The software implementation of the method is described. The practical example of multi-factor data analysis for selection of pixel size for designing large-scale aerial photography of built-up area is considered. The relevance of determining the coefficients directly in the course of aerial survey is shown.