Radar Cross Section (RCS) of space debris is generally related to space debris geometrical structure. The estimation of space debris size from RCS is investigated in this paper. We first establish an ellipsoid model. Then, by researching the RCS series of ellipsoid, it is denoted that the ratio of the number of the bigger than the mean value of RCS series to the number of the smaller is equal to the ellipsoid's curvature. And this statistical characteristics is demonstrated in the paper. Based on it, we estimate the long and short axial size of space debris. A number of practical data is processed using this method, and the estimated values are compared with the result of MIT Lincoln Laboratory's method. The result suggests that the method has a high precision, and the estimated size can provide much information to space debris surveillance.