A multiscale approach is pursued to develop a shear-lag model in combination with core-surface and core-shell models for capturing size-scale effect on mechanical properties of ZnO nanowire (NW)-reinforced nanocomposites. Surface effects are represented by a zero-thickness (finite-thickness) surface with different elastic modulus from the central part of NW. The molecular dynamics technique is utilized for calculating thickness of the shell in the core-shell model. Linear elasticity for an axisymmetric problem and the cylindrical coordinate system is used to find the closed form of governing equations. The effect of different parameters, including diameter and aspect ratio of NWs, is studied to demonstrate the application of the developed model. Numerical results disclose that NWs with a larger aspect ratio and a smaller diameter can carry a larger portion of applied stress and are preferable in designing high-performance nanocomposites. This result is in agreement with the reported computational and experimental data.