This study delves the effects of altering the geometrical dimensions of a Self-Aspirated Porous Radiant Burner (SA-PRB) of 5 kW capacity, focusing primarily on the Mixing Tube (MT) and orifice, and their influence on air entrainment and flame stability. To simulate flow and combustion, the study utilized the RNG k-ɛ and Eddy Dissipation Models, respectively, employing a four-step reaction scheme for LPG combustion. Examining various combinations of MT dimensions, orifice sizes, and their positions within the MT, the study scrutinized flame length, heat distribution uniformity, and flashback. Both numerical simulations and experimental observations corroborated the findings, highlighting the significant impact of orifice and MT dimensions on Primary Air Entrainment (PA). Optimal geometrical configurations are identified as crucial for achieving the combustion stability in PRBs. Configurations resulting in unstable PRB operation such as non-uniform temperature distribution, flashback etc., have been discussed. Stable operation is achieved in the partially submerged combustion mode with an optimized setup featuring a 0.3 mm OD orifice positioned 30 mm from the datum, coupled with a 29 mm MTD. This configuration yields a PA of 79% and a maximum temperature of 979 °C, and demonstrated efficacy of the proposed design in enhancing PRB performance.