WO3 nanoparticles (NPs) of 2.64 +/- 1.11 nm, 16.6 +/- 3.7 nm, and 54.3 +/- 18.5 nm were investigated in the cross-metathesis reaction of ethylene and trans-2-butene to form propylene. Measurements of reaction rates normalized by exposed WO3 surface area showed that decreasing WO3 particle sizes resulted in increasing propylene formation rates. X-ray photoelectron spectroscopy of post-reaction catalysts showed that smaller WO3 NPs featured lower O/W surface atomic ratios, and thus higher degrees of surface unsaturation. Based on these results, high-temperature synthetic treatments aimed at increasing the degree of surface unsaturation of WO3 NPs showed significant improvements in metathesis activity when compared to baseline values. This work demonstrates that control of particle size for industrially-relevant catalysts drastically impacts reactivity.