Highly doped semiconductor nanocrystals are of great interest for applications in nanophotonics and appear as an exciting alternative for infrared plasmonics with detection and identification of molecules, covering applications in biology, medicine, air quality control, sanitary control, and safety issues. Among the main parameters that influence optical properties and plasmonic response, the nanocrystals size plays a major role. In this work, we report on the influence of the silicon nanocrystals size on the localized surface plasmon resonance obtained in n-type Si nanocrystals embedded in a silicon dioxide matrix. The size control of phosphorus-doped Si nanocrystals was achieved by using a (SiO/SiO2) multilayer architecture. In this study, the nanocrystal diameter is varied from 7 to 16 nm while the P content is kept constant at 0.9 atom %. Here, we demonstrate that the mid-infrared plasmonic absorption exhibits a redshift as well as a broadening as the nanocrystal diameter decreases from 16 to 7 nm. The plasmonic response was successfully modelled in the framework of the Mie theory, considering the Drude model and a size-dependent scattering of free carriers. The redshift of the plasmon is explained not only by a size-dependent scattering but also by a size-dependent doping efficiency. Both charge carrier mobilities and free carrier densities are found to vary between 17 and 28 cm 2 V -1 s -1 and between 1.89×10 20 cm -3 and 2.6×10 20 cm -3 , respectively. In this work, we shed light on the key role of the Si/SiO2 interface that needs to be optimized to reach plasmonic properties in very small nanocrystals that could support quantum plasmonics.