The object of research is expanded clay fiber-reinforced concrete based on polypropylene fiber. Dispersed reinforcement with polymer fiber is one of the priority directions for modifying lightweight concrete, in particular, expanded clay concrete. There is no consensus among researchers on the effect of the reinforcement with polypropylene fiber on the strength characteristics of lightweight fiber-reinforced concrete. The influence of the cross-sectional dimensions of the specimens on the values of the compressive cube strength of expanded clay fiber-reinforced concrete has not been studied. Method. The article presents the preliminary results of experimental study of the influence on the compressive cube strength of expanded clay fiber-reinforced concrete by the following factors: the reinforcing polypropylene fiber content (0.12 %, 0.24 %, and 0.36 % by concrete volume), the edge size of the testing cubes (100 mm or 150 mm). Results. The obtained empirical data and analytical review allowed to state, that it is not ensured to obtain correct values of compressive cube strength if cube specimens are with an edge of 100 mm regardless of the reinforcement percentage. Therefore, it is recommended to test cubes with an edge of 150 mm or more. Conclusions. The characteristic values of the strength of expanded clay concrete and expanded clay fiber-reinforced concrete can be obtained on the basis of the test results of both cylinders and cubes. The mean values of the cylindrical and cube strength are comparable.