In the hierarchical approach of constitutive modelling, the damage accumulation and resulting strain-softening behavior of geologic materials can be modelled by introducing a concept of damage evaluation. The damage accumulation and softening behavior are affected by the size of specimens. A series of uniaxial tests on cylindrical specimens of a simulated rock were performed. The effect of specimen size on the parameters of the damage model were investigated by using the test results. The empirical results show that the damage parameters can be related to a characteristic dimension, which was expressed in terms of length and diameter of the specimen.