A simplified Coulomb explosion model is presented for the analysis of the explosion dynamics of hydrogen clusters driven by an ultrashort intense laser pulse. The scaling of the proton kinetic energy with cluster size has been studied in detail based on this model. It is found that the maximum kinetic energy the protons acquire in the laser-cluster interaction rises to a peak and then decreases slightly as the cluster size increases, which can be explained very well by investigating the temporal evolution of outer ionization rate of different-size clusters. It is also indicated that there exists an optimum cluster size to maximize the proton energy for given laser parameters. Moreover, taking the cluster-size distribution as a log-normal function distribution into account, the maximum proton energy increases sharply with the cluster size and then levels off before beginning to fall slowly. The inclusion of a cluster-size distribution into the simulations considerably improves the fit with experimental data. These discussions are useful for the optimum-match determination of laser-cluster parameters to obtain the maximum proton energy in experiments.