CoxZn1-xFe2O4 (0.5 ≤ x ≤ 1.0) nanocrystals have been synthesized by conventional and a modified size-controlled coprecipitation method and characterized by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometry, and Mössbauer spectroscopy. The control of the oversaturation conditions in the reacting solution, favored by the control of the flow rate at which the reactants were contacted, promoted heterogeneous nucleation and hence crystal growth, as evidenced by XRD measurements. The size-controlled synthesis route also allowed the tuning of the maximum magnetization and the coercivity, which increased by a factor of five. The demagnetization temperature was found to be strongly dependent on the Co-content and crystal size and varied between 106 °C and 540 °C. Mössbauer spectroscopy confirmed the strong influence of the synthesis conditions on the superparamagnetic fraction in the samples.