Bioelectrochemical systems (BESs) have emerged as a promising technology with applications in wastewater treatment, energy recovery, and chemical synthesis. However, several challenges, including the optimization of electrode materials and geometries, microbial community enrichment, reactor design and scale up, have delayed the transition of these technologies out of the laboratory to larger industrial process. The goal of the present work is to study the effect of electrode size of anodic biofilms and substrate concentrations. The anodic biofilms were enriched on electrodes under two conditions: 1) in raw domestic wastewater and 2) in domestic wastewater supplemented with 20 mM acetate. After enrichment, the media were replaced with various COD values to test the effect of COD loading on current density at pseudo-steady-state conditions. We demonstrated how current densities of various electrode sizes depend on the bulk COD concentration after enrichment of anodic biofilms.