In this work, a commercial activated carbon (AC) impregnated with iron (7 wt % Fe on Fe/AC) was used as catalyst for the removal of the azo dye Orange II (OII) by the heterogeneous Fenton-like process. The influence of the particle size of the AC support on adsorption and catalytic runs was evaluated, using different particle sizes below commercial pellet form (cylinders of ca. 3 x 5 mm). The materials were characterized using several techniques (N-2 and CO2 adsorption, XRD, and HRTEM). It was found that the porosity of the extruded AC was liberated by milling, enhancing the adsorption capacity and the adsorption rate. Nevertheless, total discoloration was achieved with the four particle sizes tested, albeit taking ca. 24 h using pellets but only 2 h using the powder, under the tested conditions. The dispersion of iron in the Fenton-like catalysts was also improved with the decrease in AC particle size, which also favored the catalytic activity. However, leaching increased in the same way. This tradeoff between activity and stability points to the intermediate size of 0.80-1.60 mm as the best choice. To analyze the effect of the support particle size on the catalytic performance alone, catalysts were prepared with nearly the same iron dispersion (keeping the loading of iron per unit of surface area constant). The smaller the support, the higher the activity, and thus, the higher the effectiveness factor, because of the competition between the internal diffusion and reaction rates.