The object of research is expanded clay concrete reinforced with polymer fiber made of polypropylene. Dispersed reinforcement with polymer fibers is one of the priority directions for modifying lightweight concrete, in particular, expanded clay concrete. The article presents the influence of the binary variability of the key factor (edge size of 100 and 150 mm of cube-shaped specimens) on the values of the compressive cube strength of expanded clay fiber-reinforced concrete. Methods. The article presents the experimental studies of the influence of the reinforcing polypropylene fibers content (1.5 % by cement weight) and edge size of cubes (100 mm or 150 mm) on the compressive cube strength of expanded clay fiber-reinforced concrete. Results. Even distribution of fiber throughout the volume provides the effect of crack stopping, regardless of the fiber and concrete type. However, this effect does not appear at small volumes of concrete (in cubes with dimensions of 100×100×100 mm). The empirical data allowed us to state that tests on 100 mm edge cube specimens may produce incorrect values of compressive cube strength. Therefore, it is recommended to test cubes with an edge of 150 mm or more. Conclusions. The results of testing cubes with an edge of 150 mm and 100 mm of lightweight fiber-reinforced concrete demonstrate fundamentally different effects, and testing cubes with an edge size of 100 mm does not guarantee obtaining correct results.