<p>The physical properties and mechanical behavior of rocks are fundamentally governed by their texture and mineralogical composition. This study presents a comprehensive experimental analysis of the textural, mineralogical, petrographic, and mechanical-physical properties of the Upper Red Formation sandstones. It specifically investigates the coupled effects of grain size and grain packing on the strength characteristics of selected samples, as determined by uniaxial compressive strength (UCS), mode I fracture toughness (K_Ic), point load strength (I_s(50)), Brazilian tensile strength (BTS) and triaxial strength tests. For this purpose, four sandstone groups with distinct grain sizes were sampled from northwestern Iran and subjected to a suite of petrographic, physical, and mechanical tests. The results reveal a non-monotonic relationship between grain size and sandstone properties: grain packing and mechanical strength (UCS, BTS, K_Ic, P-wave velocity) first increase and then decrease, while porosity follows an inverse trend. This pattern identifies an optimal grain size that yields maximum grain packing, minimum porosity, and consequently, peak strength. Denser sandstone fabrics, which minimize stress concentrations at grain contacts, provide superior resistance to brittle fracture under uniaxial and tensile loading. The variations of all key mechanical and physical parameters with grain size are strongly captured by second-order polynomial regressions, confirming the presence of this non-monotonic relationship. In contrast, triaxial tests revealed a consistent decrease in strength with increasing grain size. This inverse relationship is attributed to a shift in the failure mechanism to pervasive grain crushing and cataclastic flow, where larger grains are more susceptible to fracture under high confining pressure.</p>