Cellulose nanocrystals (CNCs) are sustainable rod-shaped nanoparticles able to adsorb at oil–water interfaces to produce highly stable Pickering emulsions with enhanced mechanical properties. Polymerization of the CNC-stabilized monomer droplets is investigated in detail to elucidate the synthesis mechanism of both micro- and nanoparticles of latex in relationship with the initial sizes of the droplets. It is shown in this study that unmodified CNCs, used as a sole stabilizer, are efficient to produce both monomer-in-water Pickering emulsion and nanocomposite latex particles with controlled dimensions. For the initial liquid emulsion of styrene, two populations of droplets were observed: micrometric droplets with diameters that decrease down to 5 μm with increasing CNC concentrations, and nanometric droplets with a diameter distribution ranging from 500 nm to 2 μm for all CNC concentrations. It leads to two distinct populations of polystyrene latex particles by polymerization in aqueous dispersed media, i.e. microparticles of 5–18 μm and nanoparticles with an average diameter below 1 μm. The polymerization of various monomers, i.e. styrene, lauryl methacrylate, isobornyl acrylate, butyl methacrylate, and methyl methacrylate, in the presence of different initiators indicated that the solubility of the monomer in the aqueous continuous phase is the key parameter to tune the size distribution of the latex particles. Nanocomposite CNC-stabilized waterborne latexes were prepared from polymers with different glass transition temperatures.