Marine facies of carbonate and siliciclastic sediments deposited on top of the upper Devonian Alamo Breccia Member identify the shape and size of the Alamo impact crater in south-central Nevada (western USA). There are 13 measured sections that record peritidal to deep-subtidal deposition across the impacted platform, and these are correlated to three regional depositional sequences above the Alamo Breccia Member. Facies and accommodation patterns identify a concave seafloor that we interpret as the post-impact legacy of the Alamo crater. Together with isopach and lithostratigraphic trends in the underlying Alamo Breccia Member, a new map of the Alamo crater is presented showing the eastern outer rim fault and the annular trough. Size estimates were made using the newly defined crater features and linear scaling relationships from other marine-target complex craters. Revised dimensions of the Alamo crater place its transient diameter between 37 and 65 km, and its apparent diameter between 111 and 150 km. These estimates are more than double previous estimates based on the biostratigraphy of the Alamo Breccia Member. If correct, these new estimates place the Alamo crater as one of the largest marine impacts of the Phanerozoic, and conservatively larger than the well-studied Eocene Chesapeake Bay crater.