The dependence of the maximum energy of protons Emax produced in the interaction of an intense femtosecond laser pulse (60 fs, 790 nm, 2×1016 W/cm2) with hydrogen clusters in a gas jet on the square of the cluster radius r2c has been investigated experimentally. The results obtained have established a Emax～r2c relation such as Emax(keV)=1.2+0.32r2c (nm2) with rc ranging from ～1 to 3 nm, in quite good agreement with the reported simulation results, demonstrating that the hydrogen clusters were purely Coulomb exploded under the present experimental conditions. The results also imply that the cluster size distribution in a gas jet should be taken into account in a further simulation study.