Metal nanoshells, concentric nanoparticles consisting of a dielectric core and metallic shell, have attracted much attention due to their inherently tunable optical plasmon resonances. We seek to extend the nanoshell plasmon resonance into mid-IR wavelengths by exploring “mesoscopic” nanoshells, particles with micron-scale sizes. By employing Mie scattering theory, we calculate the plasmon resonance wavelengths of nanoshells in this size regime with various values of the core radius and shell thickness. We observe the effect of phase retardation in nanoshells of increasing size, which is manifested in the appearance and increasing strength of multipolar plasmon modes. We also report a phase-retardation dominated regime where the plasmon resonance scales linearly with the nanoshell size. Finally, we demonstrate experimentally that mid-IR nanoshells can be achieved by fabrication of mesoscopic nanoshells and that their optical response agrees with that predicted by theory.