The effects of grains and sampling sizes on macro/micro-mechanical properties and the representative elementary volume of intact rocks were determined using a Voronoi element-discrete element method approach. Voronoi element was used for generation of grain assembles of rock matrices and the discrete element method was used as the numerical tool. Micro-mechanical parameters of Voronoi models were determined using back-calculation techniques through numerical calibrations. A series of two dimensional Voronoi element with different grain size distributions for calcite based marble intact rocks of Neiriz and Baghat from two quarries in Iran with varying model sizes were generated and numerical uniaxial compression tests were performed to calculate the equivalent macro-mechanical properties of intact rocks using the universal distinct element code. The results show high size-dependency of mechanical properties of intact rocks. The side lengths of 10cm and 6cm were determined as the representative elementary volumes for intact rocks of Neiriz and Baghat with grain sizes of 2mm and 4mm, respectively. The results also show that the model scale has more significant impact on macro-mechanical properties than the micro-structure pattern.