We performed long-time DP-MD simulations of DSG without an added proton in order to study the thermal fluctuations of DSG as a function of system size. We tested system sizes that spanned the range from 648 to 28800 atoms. We observed that DSG exhibits smaller out-of-plane fluctuations compared to SSG or graphane. The rigidity of DSG can be explained by the constraining effect of the hydrogen bonding networks on both sides of the material.