Glass RPC detectors are an attractive candidate for the active part of a highly granular digital hadron calorimeter (DHCAL) at the ILC. A numerical study, based on the GEANT3 simulation package, of the performance of such a calorimeter is presented in this work. A simplified model for the RPC response, tuned on real data, is implemented in the simulation. The reliability of the simulation is demonstrated by comparison to existing data collected with a large volume calorimeter prototype exposed to a pion beam in an energy range from 2 GeV to 10 GeV. In view of an optimization of the readout pitch, a detailed study of the energy and position resolution at the single hadron level for different read-out pad dimensions is presented. These results are then used in a parametric form to obtain a preliminary estimate of the contribution of DHCAL to the reconstruction of the energy flow at the ILC detector.