Nanocomposites made of elastoplastic matrices reinforced with elastic nano-inclusions exhibit enhanced mechanical performance due to size-dependent phenomena, commonly referred to as “size effects.” In this study, we develop a finite element (FE) model in a 2D plane strain framework to investigate the influence of inclusion size on the nonlinear response of these materials. Building on the work of Bach et al., who illustrated the performance of interface element approaches in capturing size effects in nanocomposites, our modeling strategy employs interface elements to represent a coherent interface between the matrix and the nano-inclusions. Indeed, usually, size effects are incorporated via a surface elasticity formulation based on a generalized Young–Laplace framework. A representative volume element (RVE) composed of an elastoplastic matrix with a fixed inclusion volume fraction is subjected to various loading conditions under periodic boundary conditions, and three inclusion radii (1 nm, 5 nm, and 50 nm) are investigated to assess both the global stress–strain response and the local stress distributions. While the macroscopic response shows only slight variations with inclusion size, significant effects are observed in the local stress fields, particularly at the matrix–inclusion interfaces and within the inclusions themselves. These local phenomena highlight the critical role of the nanoscale dimensions of the inclusions in the design of nanocomposite structures.