Size and morphological characteristics of particles play vital role on the shear and interfacial shear strength of sands. Often, effects of these parameters are merged and cannot be easily separated. Effect of size of the particles on the shear and interfacial shear strength of sands is presented in this paper through direct shear and interface direct shear tests complemented with image analyses and surface roughness studies. To eliminate the effect of morphological characteristics, three sands of different particle sizes with similar morphological characteristics like angularity, roundness, sphericity and roughness were selected for the study. These morphological characteristics for all three sands were determined from the analysis of scanning electron microscope images and were found to be similar for all three sands. It was observed from the symmetric direct shear tests that the particle size has no effect on the peak friction angle when the tests were carried out at same void ratio. However, ultimate friction angles were affected by the particle size. Shear band thickness was estimated from image segmentation analysis of the profiles of colored sand columns during shear and the same was correlated to the particle size. Interface direct shear tests were carried out on sand–geomembrane interfaces to study the effect of particle size on the interfacial shear strength. Microscopic images of geomembranes were captured after the interface shear tests to understand the change in surface roughness of the geomembrane due to particle indentations. Surface roughness studies on geomembrane samples after the tests confirmed that the plowing and groove formation on geomembranes during interface shear tests depend on the particle size as well as the relative roughness of the sand particles with respect to the membrane. Sand of medium particle size showed highest interfacial strength because of more number of effective contacts per unit area of the interface.