Nanoindentation is a micromechanical technique utilised for the characterization and measurement of hardness and elastic modulus at small scale. However, it is important to note that this approach is susceptible to demonstrate size and rate dependency. The seminal work of Nix & Gao (J. Mech. Phys. Solids., 1998), laid the foundation for quantifying indentation size effect (ISE). Several groups around the world have continued to explore the various factors that influence ISE, through extensive numerical and experimental studies. In this work, a unified approach to quantify ISE is presented, wherein the effects of material, geometry and other coupled parameters on ISE are incorporated to derive a simple unified analytical model. The analytical model is based on superposition of stress resulting from microstructural and geometric features having different length scales. To assess the analytical model, Finite Element Analysis (FEA) of indentation is also performed. The FE model includes strain gradient effects and dislocation density evolution based on the well-known Kocks-Mecking-Estrin (KME) model implemented as a user material model (UMAT). The analytical model predictions and the FEA results are validated by uniaxial compression and indentation tests (nanoindentation and micro hardness) on nickel. Insights on the relative contributions of the various microstructural and geometric parameters will be presented. In addition, the well-known concept of equivalent strain during indentation will be revisited in the context of ISE. Hence as a preliminary approach, a quantitative correlation is established to quantify the size effect which will subsequently in the future be extended to include the influence of strain rate as well.