Static and dynamic magnetic properties of Zn0.4Fe2.6O4 nanoparticles synthesized by thermal decomposition have been investigated. Two ranges of diameter have been used: small particles (diameter about 6.2 nm) and larger ones (diameter about 22.4 nm). The nanoparticle microstructure has been characterized by transmission electron microscopy. The temperature dependence of the zero-field dynamic permeability for both nanoparticle sizes has been studied, revealing a superparamagnetic state for the small ones. Effects of the nanoparticle size on the dynamic permeability have been studied, and linked to the superparamagnetic state. A dynamic susceptibility model has been found to reproduce the experimental behavior as well as its temperature dependence.