Low-velocity impact-induced reaction of two high explosives (PBX-1 and Composition B) with four different specimen size designs was experimentally studied. The reaction threshold velocity, delay time of visible reaction, pressure history, and reaction violence under impact were measured. The experimental results indicate that the reaction threshold velocity and the reaction violence have a significant specimen size effect. In the range of tested diameter, the threshold velocity increases with the specimen thickness for PBX-1, but decreases with the specimen thickness for Composition B. The results show that the reaction violence increases with the thickness but decreases with the diameter of high explosive charges. In addition, the mechanisms of the size effect are discussed.