A method for accurate measurement of the size of objects from X-ray diffraction enhanced imaging (DEI) and computer tomography was proposed. X-ray DEI is an imaging method based on phase contrast. A simple model of DEI was set up, by which the relationships among crystal angle, valley positions in image, and system blurring were studied. The deviation of projection valley points caused by the blurring effect was discussed, and a new method to measure diameter of a round sample from projective DEI profile was proposed. DEI data from both simulation and Beijing Synchrotron Radiation Faculty ware processed to demonstrate the validity and accuracy of the method. This method presents an accurate measurement method of the size for objects, especially for the inner structure of small biological tissues or samples.