<p>Interfacial shear characteristics of geocell-reinforced backfill are critical to stability in reinforced retaining wall structures. The influence of size-related factors (i.e. geocell equivalent diameter <i>d</i>, height <i>h</i> and aspect ratio <i>h/d</i>) on the shear behavior of geocell-sand interface was explored using a three-dimensional discrete element method (DEM). DEM models of geocell-sand interfaces with different specifications were validated against experimental data. Results show that strains at welded junctions of geocells are generally greater than those at the strip sections, and larger equivalent diameters of geocells correspond to lower strains. Within the range of variables in this study, changes in geocell diameters and heights influence the magnitude of mean contact forces by 5.33% and 3.14%, respectively, while their impacts on the principal direction of contact force are measured at 4% and 2.7%. Additionally, aspect ratio (<i>h/d</i>) significantly affects the location of the maximum normal contact force between geocell and sand. For the geocells with <i>d</i> ≥ 170&#xa0;mm, their sidewalls exert a relatively weak confining effect on the internal soil particles within a range of 1/3<i>d</i> from the cell center. Finally, the geocell-sand interface shear strength was predicted by two machine learning models.</p>