Dedicated research on photochemical reactors is fundamental to the application of this technology to real-life problems. In line with this trend, we report a study of structural parameters of micro-structured packed bed reactors (μPBR) regarding their ability to promote intensification. We present an investigation on the effects of the size of spherical packings on the developed flow regime, and its implications for the performance of the reactor on a photochemical reaction in aqueous phase. Tubular reactors were built with combinations of borosilicate glass tubes of two diameters (20 and 30 mm) packed with soda-lime glass spheres of three average diameters (1.0, 3.0 and 6.0 mm), alumina spheres of 6.0 mm diameter, and polished iron spheres of 5.0 mm diameter. Residence Time Distributions (RTD) were measured for each combination of outer tube and glass sphere sizes. Results showed that as the packing size decreases, the observed flow regime becomes closer to an ideal PFR, suggesting that smaller packings are more suitable to applications that require precise control of residence time. The aqueous-phase oxidation of benzoic acid by oxygen radicals generated by UV-A irradiation of nitrite ions was used as model photochemical reaction. We also observed that increasing the D/dp ratio for narrower reactors faster specific reaction rates are achieved, suggesting that intensification is possible in these reactors; however, the overall yield decreases, probably due to increased photon loss. Lastly, we found that glass spheres can be replaced by alumina spheres with little loss in light-collection efficiency, indicating an attractive pathway for immobilized photocatalysis.