This article defines biggest rock face as the total vertical gain of a climbing route. It’s not El Cap; “…contrary to popular lore, it’s not the Lower 48’s biggest rock face. It’s not even the biggest in the Valley – the south face of Mt. Watkins is bigger.”
Some other tall rock faces:
- South face of Mt. Moran, Grand Teton National Park, Wyoming
- South face of Tehipite Dome, Kings Canyon National Park, California
- Johannesburg Mountain, Cascades, Washington
Read about the criteria used and the actual vertical gain in the article here.