With regard to high water head and large flow velocity in the spillway tunnels of hydraulic projects in China, the aerator device has been introduced and is widely used to prevent cavitation damage. The bottom rollers in the nappe cavity below the aerator device are a serious concern in designing suitable cavity regimes; however, observation of roller size may be inaccurate due to high flow turbulence and de-aeration in the jet impact region. In this study, a novel approach is proposed to predict roller sizes using pressure distribution of the bottom rollers. Pressure distribution characteristics are experimentally investigated under different geometrical parameters of aerator device and hydraulic conditions. The results specify the influence of the relative step height and working gate opening on pressure distribution. The simplified estimating formula of pressure distribution is derived within relative errors of 15%. The evaluation of the applicability of the proposed equation shows test data are in good agreement with the calculated value. Research results provide a reference for estimating bottom rollers of similar engineering.