[en] Different-sized Co-ferrite nanoparticles synthesized from nanophase cobalt-iron hydroxide carbonate precursors in an inverse microemulsion system have been studied by using X-ray diffractometry, transmission electron microscopy, and zero- and in-field M¨ossbauer spectroscopy. The lattice constant a0 tends to decrease with decreasing particle size. At liquid-helium temperature, the magnetic hyperfine field at each site decreases with decreasing particle size. The particle size dependence of the site distribution and the superparamagnetic relaxation frequency is also examined.