This paper addresses the effect of support pore sizes on the structure and performance of iron catalysts supported by mesoporous silicas in higherature Fischer-Tropsch synthesis. A combination of characterization techniques showed that the size of supported iron particles was controlled by catalyst pore sizes. The larger iron particles were localized in large-pore supports. Iron carbidization with carbon monoxide resulted in preferential formation of Hägg iron carbide (χ-Fe5C2). Larger iron oxide crystallites in large-pore supports were much easier to carbidize than smaller iron oxide counterparts in small-pore supports. The catalytic performance in Fischer-Tropsch synthesis was attributed to iron carbide. Higher Fischer-Tropsch reaction rates, higher olefin, and C5+ selectivity were observed over larger pore iron catalysts. High dispersion of iron oxide in small-pore silicas was not favorable for carbon monoxide hydrogenation because of poor iron carbidization.