Small-scale experiments are often used to investigate rock fracture and stability but sometimes verified to be some non-ignorable deviations while compared with full-scale actual projects. To settle this difficulty, a new dimensionless basis, DLW (material density, crack half-length and transverse waves velocity) basis, is presented to study the size effect under impact load. The advantage and validity offered by this new method are demonstrated numerically in the paper with a focus on mode-I fracture toughness of SCSC sandstone specimen. It should be stressed that the errors are quite large between prototype and scaled models when the traditional dimensional analysis is applied. Therefore, an adjustment to the impact stress in the scaled models is made to ensure fewer deviations according to the similitude conditions. It is shown that errors will be significantly reduced by means of impact stress correction. Additionally, it is revealed that the impact stress correction factor increases linearly with scaled model size ratio, which is useful to directly modify the scaled models to obtain rock fracture toughness, and helpful to predict full-scale rock fractures via small-scale experiments under impact load.