Laser speckle imaging technique, based on laser speckle contrast analysis, is a full field optical imaging method which does not need scanning. It is getting increasing use in monitoring dynamic fluctuation of blood flow in tissues in physiological and pathological situations. In practice, various factors such as averaged image speckle size have impact on speckle contrast, making the accuracy of this technique affected in reflecting blood flow changes. This study investigates the impact of averaged image speckle size on statistical characteristics of image speckle pattern by simulating image speckle pattern. The quantitative relation between imaging parameters and averaged image speckle size is analyzed. The results are finally validated by phantom experiment. These results confirm the reasonable formula for calculating averaged image speckle size and the phenomenon that speckle contrast increases with increasing averaged image speckle size, providing a basis for determining reasonable imaging parameters.