The performance of vanadium redox flow batteries (VRFB) is affected by multiple factors, such as flow field design and electrode size. Providing a smooth electrolyte supply to a suitable electrode size is determinant for battery performance. In this work, the dual impact of channel height and electrode thickness on overall battery efficiency (in nine different combinations) was investigated numerically through a 3D model involving electrolyte motion. The individual role and the dual impact of both factors on electrolyte penetration into the electrode, overpotential, and pressure losses were evaluated to highlight how these affect the output charge-discharge voltages, optimal flow rate, energy efficiency, and overall system efficiency. Numerous operating conditions, like state of charge (SOC), volumetric flow rate, and current density, were implemented to identify the most efficient cell combination among the studied cases. In general, the voltage response is improved as the channel height is reduced and/or electrode thickness is increased due to the reduction in overpotential; however, large pump losses are also produced. A balance between enhanced voltage and pumping power increment is required to achieve maximum battery efficiency (at optimal flow rate), depending on the cell configuration and applied conditions. Results indicate that channel height has a positive effect on energy efficiency at low flow rate, more so than electrode thickness, whereas the opposite is noticed at high flow rate. Additionally, for the best battery efficiency, electrode thickness is more determinant, followed by channel height, based on operating conditions. Overall, case 7 (smallest channel height and largest electrode thickness) performed best in terms of energy/battery efficiency at a relatively low optimal flow rate, whereas case 3 (largest channel height and smallest electrode thickness) performed worst; however, this effect is lowered at high current density. This model can guide the design of optimal flow batteries and loading conditions.