A focal spot size is an area on the source of radiation from which radiation is emitted. Determination of the focal spot size, as well as her characteristics, is crucial for determination of geometric unsharpness and it is done in several different ways, from which nine are mentioned in this paper. The idea on which this paper is based on was to check and explore the possibilities of characterization of focal spot in terms of estimating the size of the focal spot calculated by previously determined geometric unsharpness. Geometric unsharpness represents loss of image sharpness and the aim is to make its value as small as possible, and therefore also minimalize the focal spot size. As part of this work three methods were used, whereby the first one was the one that wants to be implemented, while the other two methods were used as supervision for the first method. For duplex and pinhole methods, four different geometric setups were done, while for the third one that uses ERESCO x-ray generator, only two were made. In those setups, distances from an object to a source and from the object to a film were being set at the certain values to get more calculations for assessment purposes. The primary method, that uses duplex indicators, was used to calculate geometric unsharpnes, after which it was possible to assess the value of the focal spot size. With small modification of this method, received results were more precise than those prescribed by the norm.