Herein, based on the hydrodynamic Drude model and two size-corrective terms derived from the size-dependent dielectric function, a simplified dielectric theoretical model is proposed to describe the nonlocal and size-dependent effects of surface plasmons in metallic nanostructures. Different dielectric models are used to perform numerical simulation comparisons of the electron energy loss spectroscopy and optical responses for silver spherical nanoparticles with different radii of 1-100 nm. Results show that the proposed theoretical model can demonstrate great effectiveness in reflecting the effects of local, nonlocal, size-dependent effects, and even analogous quantum-size responses on the surface plasmon characteristics of metallic nanostructures, energy (1-5 eV), and size (2-200 nm) ranges. Simultaneously, these results are useful in understanding the resonance mode and energy distribution and dynamic evolution mechanisms of surface plasmons, and provide a reference for the development and design of plasmonic elements on the nanometer scale.