The influence of the precursors, namely potassium ferrocyanide and potassium ferricyanide on the particles sizes of Prussian Blue (PB) and Prussian Green (PG), under identical reaction conditions have been investigated. It was found that the particle sizes influence the gravimetric capacity utilization of these materials as cathodes for aqueous potassium (K+) ion batteries. The PG particle sizes were on the order of 50–75nm, whereas PB particles size were on the order of 2–10 microns. The PG cathodes demonstrated a reversible capacity of 121.4mAhr/g, with a coulombic efficiency of 98.7% compared to PB cathodes which demonstrated 53.8mAhr/g, with a coulombic efficiency of 100%. We interpret the increased capacity of PG batteries relative to PB batteries as being a result of the smaller particle size of PG, which results in greater accessibility of the cathode to K+ ions.