<p>The mechanical integrity of masonry systems is governed by the complex interfacial bonding between the unit and the binder. This study presents an experimental investigation into the shear and bond strength of clay-gypsum composites, with a specific focus on the scale effect—a critical factor in predicting the performance of small-scale prototypes versus full-scale structures. Using brick units ranging from full-scale to one-sixth scale, the research quantifies how specimen size influences fundamental mechanical properties. Experimental data reveals a significant inverse relationship between scale and strength: as the characteristic dimension decreases, the compressive, flexural, shear, and bond strengths exhibit a consistent increase. This paper provides empirical scaling laws derived from these results, alongside a proposed constitutive formula to determine bond strength directly from flexural strength measurements. The findings demonstrate that bond strength remains 40–60% higher than flexural strength across all scales, providing essential data for the development of accurate size-dependent mechanical models for brittle composite systems.</p>