We have conducted a comprehensive series of experiments to evaluate the effects of pore size distribution of porous media on the dynamics of shear-thinning fluid flow and oil displacement efficiency. To do so, we have conducted microfluidic experiments, using micromodels fabricated from X-ray computed tomography images of sand-packs with varying grain sizes to create a realistic pore network. Three micromodels with well-defined particle size distribution were fabricated and used in our experiments. The use of micromodels to assess the effectiveness of polymer flooding is often superior to methods such as core flooding because of the detailed pore-scale information obtained during the experiments. The micromodels were initially saturated by oil, and the displacing fluids were prepared as aqueous solutions with dissolved xanthan gum. The dynamics and patterns of the interface displacement as well as the size distribution of the trapped oil ganglia were visualized using an optical microscope. The main findings of these experiments are that polymer flooding produces higher recoveries compared to water flooding. However, the effect of pore size on oil entrapment is not uniform across different viscosity ratios, and the relationship between particle size and the quantity of trapped oil ganglia is not consistent across the pore size ranges of the micromodels. Polymer flooding is less efficient in systems with smaller pores, where capillary forces dominate over viscous forces. These findings extend our understanding of the mechanisms controlling the effectiveness of polymer flooding for oil recovery.