Three sizes of CuO nanosheets were synthesized by hydrothermal method. The structure and morphology of CuO nanosheets were characterized by X-ray diffraction and scanning electron microscopy. Dielectrophoresis nano-manipulation technique was employed to arrange the materials on pre-designed Ti/Au electrodes to fabricate the three humidity sensors, and the sensing properties were then tested. The experimental results show that the sensitivity greatly increases with the decreasing size of CuO nanosheets, the sensitivity of sensor a, b, c are 369%, 3278%, 22,611% in 97.3% RH, respectively. The smaller sized CuO nanomaterials have better response characteristic, the response time of sensor a, b, c under 11.3–97.3% RH are 53 s, 49 s, 32 s, respectively. And correspondingly, hysteresis properties and the repeatability are also a little influenced. In addition, based on complex impedance spectroscopy and multilayer adsorption theory, the impact of size on humidity sensing property was discussed. The results indicated the feasibility to obtain higher performance of humidity sensor, especially the higher sensitivity, via employment the smaller size sensing nanomaterials. Graphic Abstract