Flow-electrode capacitive deionization (FCDI) is a potential energy-efficient electrochemical water desalination technology that combines ion-exchange membranes and flowable electrodes (FE). The performance of activated carbon (AC)-based FEs in FCDI systems is influenced by various factors, including particle size. Here, particle size commercial AC was reduced by dry ball milling to produce FEs for FCDI. Physical analysis by scanning electron microscopy and particle size analyzer confirmed the particle size reduction after dry ball milling. Then, the effect of particle size distribution on the rheological behavior and desalination performances of AC-based FEs was investigated by desalination performance indicators, such as desalination efficiency, average salt adsorption rate and charge efficiency. FE samples were prepared using different mixtures of fine-size range (0.65-0.92 µm; fine AC, FAC) and large-size range (1.5-2.3 µm; AC) particles. The AC to FAC particle ratio influenced the rheological properties of AC-based FEs in FCDI. Bimodal mixtures with 75% of FAC and 25% of AC (0.75:0.25 FAC:AC) particles displayed the highest desalination efficiency compared with pure AC and pure FAC FEs (72% versus 34% and 71%, respectively). Moreover, in our FCDI setup , 0.75:0.25 FAC:AC bimodal mixtures showed better flowing properties than the pure FAC FEs, characterized by high viscosity.