The electrochemical CO ₂ reduction reaction (CO ₂ RR) using copper‐based catalysts offers a promising pathway for energy recycling. However, controlling product selectivity remains a significant challenge. In this work, we systematically investigate the size‐dependent electrocatalytic performance of cubic Cu ₂ O catalysts. A remarkable switch in product selectivity from C ₂ H ₄ to CH ₄ is observed as the catalyst size increases. Comprehensive characterizations reveal that the larger Cu ₂ O catalyst (Cu ₂ O‐3000) exhibits superior resistance to electroreduction, thereby maintaining a high proportion of surface Cu ⁺ species. These retained Cu ⁺ species suppress C─C coupling and promotes the formation of CH ₄ . In contrast, smaller Cu ₂ O catalysts (Cu ₂ O‐500 and Cu ₂ O‐1000) undergo rapid electroreduction, forming the Cu ⁰ /Cu ⁺ interfaces that facilitate C─C coupling toward C ₂ H ₄ . Furthermore, in situ characterizations indicate that a high density of surface Cu ⁺ species, coupled with a scarcity of Cu ⁰ /Cu ⁺ sites, significantly enhances the adsorption of both OH ⁻ ions and *CO intermediates. This configuration shifts the hydrogenation pathway of *CO from forming *COH toward *CHO intermediates, thereby favoring the formation of CH ₄ over C ₂ H ₄ . Consequently, we establish a distinct structure–performance relationship between the particle size of Cu ₂ O catalysts and the surface content of Cu ⁺ species, thereby providing a viable strategy for regulating product distribution from CO ₂ RR.

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