A simple binary-solvent model is theoretically employed to study the size-effect on selective solvation. We consider multi-component hard-sphere molecules with solute-solvent attraction. We employ the PY approximation to calculate coordination numbers. The calculated coordination number shows that the solute attracts more large solvent molecules than small solvent molecules at a weak attraction. The solute, however, attracts small solvent molecules at strong attraction. To study the selectivity, we divide the size-effect into the exclude effect and the steric hindrance. Attractive strength increases the steric hindrance of large solvent molecules more than that of the small solvent molecules.