Why do shape and size have the greatest effect on the optical properties of nanoparticles?
As Surface Plasmon Resonance phenomenon is mainly seen in metal nanoparticles, I don't know whether this answer will help you? The size dependent optical phenomenon of semiconductor nanoparticles (e.g., $\ce{CdS}$) can be explained using the effective mass approximation model. 


where r is radius of the particle (the rest details of the above equation can be find in the link).

so the energy gap is inversely proportional to the radius of the particle.



When the radius of the particle  decreases, the band gap increases so it will emit high energy wavelength (i.e., wavelength in blue region) and when the particle size is large the energy gap will be less so it will emit light in red region (lesser energy).