Classical macroscopic electromagnetic descriptions fail to accurately predict the optical response of plasmonic nanostructures approaching electronic length scales due to the omission of nonlocality and free electron spill-out. Alternatively, microscopic first-principles approaches are accurate, yet too computationally intensive for multiscale problems. Starting from a microscopic viewpoint but more computationally efficient, particle-in-cell (PIC) simulation is probably an appropriate mesoscopic treatment for such problems. Here, we leverage on the inherent soft boundary conditions of PIC to study the spill-out effects of surface electrons in mesoscopic-scale metal nanostructures, specifically Na nanowires [1]. The relationship between the surface plasmon resonance frequency and the radius of the nanowire is investigated, exhibiting a 1/R redshift, which agrees well with the self-consistent hydrodynamic method [2] that includes spill-out of electron density. The developed methodology can be extended to model noble metals by including inter-band transitions, di-metallic nanoplasmonic structures, electron spill-out and charge transport between nearly touching nanoparticles.