The size effect of transition-metal nanoparticles on electrocatalytic performance remains ambiguous especially when decreasing the size to the atomic level. Herein, we report the spatial isolation of cobalt species on the atomic scale, which was achieved by tuning the zinc dopant content in predesigned bimetallic Zn/Co zeolitic imidazole frameworks (ZnCo-ZIFs), and led to the synthesis of nanoparticles, atomic clusters, and single atoms of Co catalysts on N-doped porous carbon. This synthetic strategy allowed an investigation of the size effect on electrochemical behavior from nanometer to Ångström dimensions. Single-atom Co catalysts showed superior bifunctional ORR/OER activity, durability, and reversibility in Zn-air batteries compared with the other derivatives and noble-metal Pt/C+RuO , which was attributed to the high reactivity and stability of isolated single Co atoms. Our findings open up a new avenue to regulate the metal particle size and catalytic performance of MOF derivatives.