<p>This investigation explores the influence of specimen sizes on the fracture toughness of different failure modes of cemented quasi-brittle materials that contain pre-existing cracks under pure tensile, shear, and mixed tensile-shear loading conditions and the factors that intensify specimen size effects. To accomplish this, the cracked straight-through Brazilian disc specimens of four cement mortars with the same preparation conditions were employed in four different radii, namely 25, 37.5, 50, and 75&#xa0;mm. The crack length-to-radius ratio was fixed at 0.3, and the thickness-to-diameter ratio of all specimens was fixed at 0.5. The specimens had three crack angles concerning the loading directions equal to zero, 14, and 27.2 degrees. The only difference between the mortars was the d₅₀ size of the sand particles that make up the cement mortar. The four cement mortars had average sand particle size distributions (d₅₀) of 0.33, 0.65, 1.1, and 1.5&#xa0;mm, respectively. The results revealed that the fracture toughness of different modes increases with increasing specimen’ dimensions, and d₅₀ is an effective parameter in the sensitivity of cement mortars’ fracture toughness to the specimens’ sizes. Increasing the value of d₅₀ increases the brittleness behavior in cement mortars, and with increasing brittleness index, the sensitivity of fracture toughness to the specimen’s size decreases. Also, the critical stress intensity factors were calculated analytically based on the modified maximum tangential stress criterion and compared with the experimental results in various loading modes. It is shown that the analytical results predicted by this criterion are in good agreement with the experimental results.</p>