Fractured lost circulation management is a critical challenge in drilling engineering, and existing methods often rely on empirical designs with limited effectiveness. This laboratory-scale study, utilizing modified PPT equipment and nut shell-based rigid particles in simplified linear fractures, systematically investigated the bridging and sealing effect of compounded multi-sized rigid lost circulation material (LCM) through physical experiments. It clarified the mechanism by which the proportion of multi-sized plugging particles and the total concentration influence the sealing effect after compounding. A set of quantitative compounding relationships for multi-sized rigid LCM was established based on the particle size-to-fracture width ratio (R = D50/w, where D50 is median particle diameter and w is fracture width). Experimental results show that when using particles with specific R (e.g., R = particle size/fracture width) values, approximately R1 = 0.7, R2 = 0.45~0.3, and R3 = 0.08, in a 1:1:1 ratio, and the total concentration is around 4%, high-efficiency plugging can be achieved. The plugging formulation based on this design principle can effectively seal medium-scale lost circulation fractures, providing a theoretical basis for the scientific design of rigid LCM in fractured formations.