Size dependence of physicochemical properties is relevant for the design and practical deployment of nanoparticles. Knowing these trends beforehand can aid in designing synthesis, heat treatment, and device fabrication approaches in a rather targeted and size-directed manner. Here, BaTaO2N nanoparticles, relevant for ongoing research on photocatalysis and dielectrics, are chosen. Spheres of size 2–6 nm have been taken for analysis to study the size effect in the ultra-small regime using molecular dynamics. We show that CP (specific heat at constant pressure) increases with temperature, but with additional features that become pronounced with size reduction. This is due to the role of surface contribution and formation of vacancies. Diffusion constant (D) vs. particle size curves indicate that D decreases with increasing size. This variation is found to be less for tantalum at lower temperatures (< 2000 K); this correlate well with the nature of its bond valence parameters. Results presented here have implications to the quasi-chemical defect chemistries, phase, and interfacial stability, and sintering dynamics of BaTaO2N nanoparticles. © 2019, King Abdulaziz City for Science and Technology.