One of the main advantages of small-size betatrons is small focal spot size. However there is no conventional procedure of focal spot size measurement for betatrons. Techniques described in standards are suitable only for low- and mid-energy tubes. The task become more complex if the focal spot size should be estimated quickly using modern digital detectors. In this paper we present preliminary results of focal spot size measurement of a 4 MeV small-size betatron with two different procedures using digital x-ray detector.