Display omitted •The CO2RR on precise number of Fe atoms is systematically investigated.•Fe dimer and Fe trimer exhibit the highest catalytic performance toward CO2-to−CH4 and CO2-to−HCOOH conversion, respectively.•The catalytic activity and selectivity can be significantly tuned by controlling the number of Fe atoms. Single atom catalysts have become a hot frontier in heterogeneous catalysis due to the low cost and high catalytic efficiency. However, a general question is whether a single atom active site is the most optimal to deliver the highest catalytic activity and selectivity or not? By using ab initio studies, herein we report a systematic investigation of the active-site dependent activity/selectivity for CO2 electrochemical reduction over a few Fe atoms (Fen, n = 1–4) doped graphdiyne. We find that Fe dimer and Fe trimer exhibit the highest catalytic activity and selectivity with the remarkably low rate determining step of 0.29 and 0.35 eV toward CO2-to−CH4 and CO2-to−HCOOH conversion, outperforming many reported catalysts to date. Moreover, the catalytic activity and selectivity can be significantly tuned by controlling the number of Fe atoms. The modulation of performance is attributed to the broken linear scaling relationship on different active-site structures that can significantly tune affinities to the key intermediates such as *CO, *CHO, *OCHO and*OCHOH, thus leading to efficient CO2 reduction over catalyst size. Our work is the first report of catalyst size effects for precise number of atoms (Fe1-4) which may open a new avenue for nanocatalyst design for electrochemical reduction of CO2.