<p>The paper discusses methods for estimating the grain size of polycrystalline materials based on the analysis of optical microscopic images, as well as the possibility of applying these methods to the analysis of rock textures. A comparison of the results obtained using several methods on a single sample of metamorphosed sandstone from the Northern Ladoga region is presented. For this purpose, the manual grain size determination method, the linear intercept method, and the method of randomly point hitting with construction of oriented reference segments were used. The obtained estimates of grain sizes were compared with each other and independent broadband acoustic spectroscopy data. For certain grain size distribution parameters (first of all, the average grain size), all the methods yielded similar results. However, for other parameters, the obtained data can vary significantly.</p>