In summary, we have demonstrated that precise control over the reaction conditions during nanocluster (NC) synthesis allows for fine-tuning of the structural architecture, which significantly impacts the catalytic properties of a series of Cu NCs. Our findings underscore that subtle variations in reaction parameters validate the critical role of surface ligands in determining the overall geometry while highlighting the influence of core confinement in governing catalytic behavior. Specifically, we observed that the non-confined Cu5 core in Cu11PTT NC exhibits superior selectivity for HCOOH during electrochemical CO2RR. In contrast, the Cu18 NC, which features a larger but more confined core structure and a similar ligand environment, demonstrated reduced selectivity for HCOOH under identical reaction conditions. This outcome highlights core confinement as a decisive factor influencing product selectivity. Furthermore, when the hydrophobic surface ligands were replaced with hydrophilic ligands, coupled with enhanced core confinement, a notable shift towards competing side reactions was observed. Thus, this study reveals a complex interplay between core geometry, core confinement, and surface ligand properties in dictating the catalytic performance of Cu NCs.