This study evaluated the effects of specimen and coarse aggregate sizes on the mechanical properties (compressive strength, modulus of elasticity, and Poisson’s ratio) of concrete. Cylindrical specimens of three concrete grades (C25, C45, C60) with two different coarse aggregate sizes (16 and 25 mm), and four specimen sizes (0.66 × 10 ³, 1.57 × 10 ³, 5.30 × 10 ³, and 12.56 × 10 ³ cm ³) were tested under static loading. The results reveal that smaller specimens consistently exhibited higher compressive strength and modulus of elasticity, with compressive strength reductions ranging from 51.54% to 56.42% as specimen size increased. Lower-grade concrete (C25) was more sensitive to size effects, while higher-grade concrete (C60) exhibited improved resistance. The Modulus of elasticity decreased by up to 30.5%, with smaller coarse aggregates sizes. Concrete bulk density decreased with specimen size, reflecting increased material heterogeneity and void content in larger specimens. Larger aggregates (25 mm) generally resulted in higher densities due to efficient packing, except in C60, where better binder quality offset this effect. The Poisson’s ratio increased slightly with specimen size, ranging from 0.15 to 0.25, reflecting greater lateral strain in larger specimens. Smaller aggregates occasionally exhibited higher Poisson ratios, indicating improved ductility.