Modeling of tsunami waves generated by subaerial landslides is important to provide accurate hazard and risk assessments in coastal areas. We perform small-scale laboratory experiments where a tsunami-like wave is generated by the collapse of a subaerial granular column into water. The maximal amplitude of the leading wave is shown to scale linearly with the instantaneous immersed volume of grains and sublinearly with the final immersed deposit. From available measurements of the immersed deposits of past geophysical events, the maximal amplitude of the generated wave predicted by our empirical law is in good agreement with previous numerical or experimental modelings based on the inland geometry of the slide. Our results suggest that o shore deposits of past landslides can be used to constrain the amplitude of paleo-tsunamis.