An interesting question. The simple old models say that the PZC should not depend on curvature unless the size of the particles is comparable with the thickness of the adsorption layer (which is ~0.5-1 nm).

However, simple old model assume that the surface of the small and the large particles is similar. With most preparation methods, this is not the case. I can imagine a situation where the large particles are product of fast growth process and contain many defects, and the small particles are product of a slow process and have smoother surface. This will affect PZC (but perhaps not much).

Another thing that the simple models neglect is that each face of the crystal has different PZC. The PZC one measures refers to average surface charge = 0 (but each face carries a charge). If the differences are significant, it is easy to show that the average PZC will change when the size of the particles becomes similar to the Debye length, which could be several nm.

Re: what influences PZC mostly: the chemistry of the surface, the presence of ionic surface active components in the solution, the nonionic surfactants can also affect it, the chemistry of the solvent plays a role, and sometimes also the temperature. I wonder about the capillary pressure - the smallest crystals are of slightly higher solubility and this could affect the state of their surface.