Gap fraction and foliage clumping index play key roles in plant light interception; therefore, they have strong influences on plant growth and canopy radiative transfer processes. Leaves always aggregate in crowns, which are important objects innumerous geometric-optical models of forests. Researchers have mainly focused on the influences of crown shape (e.g., cylinder, cone, ellipsoid, and cone + cylinder) on gap fraction and foliage clumping index. However, size is also an important characteristic of tree crowns. Crown sizes result from the interactions between plants and environments during the long-term evolution process of the plants. In fact, crown size characteristics have more obvious geographical spatial features than crown shape. Therefore, crown size should be given considerable attention when studying plant light interception and canopy radiative transfer processes in the fields of phytogeography, remote sensing and global change. The main objective of this study is to exhibit how crown size characteristics influence the gap fraction and foliage clumping index of forest canopies. First, a simple and general distance factor, which is defined as the relative allowable shortest distance between centers of any two crowns divided by the mean diameter of tree crowns, is proposed to quantitatively describe the degree of repulsion effect among trees in forest canopies. Second, the Poisson distribution model of trees is completely replaced by the hypergeometric model, which is more suitable for quantitatively describing the repulsion effect and spatial relationship among trees in forest stands. Finally, seven sizes of ellipsoids (from prolate ellipsoids to oblate ellipsoids) are selected for the tree crowns, and the influences of crown size on gap fraction and foliage clumping index are exhibited by means of fixing the radius and the volume of the tree crowns, respectively. Results show that the following: (1) the influences of crown size characteristics are significantly greater than the effects of crown shape on gap fraction and foliage clumping index, regardless if the radius or volume of tree crowns are fixed. (2) From prolate ellipsoids to oblate ellipsoids, crown size characteristics show significant and regular effects on both gap fraction and foliage clumping index. In extreme cases, when theta=60°, the canopy gap fraction when ellipsoid height (H_b) is 16 m is 92.8% lower than that when H_b=0.25 m, and the foliage clumping index when H_b=16 m is 261.8% greater than that when Hb=0.25 m when the crown radius is fixed. At theta=0°, the canopy gap fraction when H_b=16 m is 414.1% greater than that when Hb=0.25 m, and the foliage clumping index when Hb=0.25 m is 11397.2% greater than that when H_b=16 m when the volume of crown is fixed. The larger the projected area of the crown in the observed direction, the lower value of the gap fraction and the higher value of the foliage clumping index, indicating that leaves tend to distribute randomly in forest canopies.