Digital Image Correlation (DIC), as a technique based on image processing, has been intensively used for displacement measurement. The setup of DIC analysis starting condition is crucial to the validity of the results. In this paper, the influence of correlation window size (corrsize), one of the parameters that need to be chosen by users for setup, is studied. The results show that DIC is reliable as long as the corrsize is between a lower bound (corssizemin) and an upper bound (corssizemax). The reason why DIC returns high error factor when beyond corssizemax is related to image boundary effect, which is demonstrated by an equation serving as a criterion for corrsize choice. The study in this paper is particularly vital in cases where the Region of Interest (ROI) is close to the image and the real displacement is large.