In this paper, the Reynolds equation of air damping of MEMS wine-glass resonator is solved by numerical calculation method considering the air slip boundary conditions. The corresponding pressure distribution between internal gaps is obtained by MATLAB software. And then size effects of resonator on air damping and quality factor are analyzed. It is found that MEMS disk radius and thickness have slight effects on the change of the pressure distribution. But the effects of spacing between disk body and electrodes are very significant. The designed ratio value of disk amplitude and spacing should not be over 70%, otherwise the changes of air pressure between gaps will be very remarkable and increased exponentially, the air damping will begin to increase doubly, and the quality factor will be reduced greatly. The analyzed results are very meaningful for optimizing design size of MEMS disk resonator.