Metal–organic framework (MOF)-derived carbons, known for their highly tunable structures, have attracted considerable attention for electrochemical applications. Efficient ion and electron transport, along with low electrode resistance, is critical for enhancing performance in these areas. To optimize MOF-derived carbons, we synthesize Zn-based zeolitic imidazolate framework (ZIF-8) nanocrystals with controlled sizes and a narrow size distribution, resulting in nanoporous polyhedral carbon structures. The sample is then subjected to carbonization to yield ZIF-8-derived carbon (ZIF-8-C) doped with heteroatoms, and subsequently, performance evaluations of supercapacitors are conducted to assess their ion and electron transport properties. Larger particles exhibit greater capacitance loss at high scan rates or current densities, likely due to underutilization of pores for ion diffusion. Uniform particle sizes facilitate ordered packing, improving electron pathways compared to electrodes with non-uniform particles and yielding higher electrochemical performance despite similar specific surface areas. Notably, the electrode prepared with the smallest and most uniformly sized ZIF-8-C-m1 exhibits a specific capacitance of 206.4 F g−1 at 1 A g−1, along with excellent rate capability and stability, retaining 99.7% of its capacitance after 10000 cycles at 10 A g−1. In a two-electrode system, it achieves an energy density of up to 19.4 W h kg−1 at a specific power of 350 W kg−1. The results present here offer valuable insights into the utilization of nanoporous carbons across diverse electrochemical applications.