Answer:
The largest volcano on Earth is Mauna Loa, Hawaii, with an area of 5271 km2, more than half of the "big island" of Hawaii. It is also the largest in volume, at more than 75,000 km3. The Mauna Loa (long island) ridge extends for more than 120 km, but at a slightly lower elevation than Mauna Kea to the north.
The largest volcanic crater in the continental US is the Yellowstone supervolcano in Yellowstone National Park, in NW Wyoming in the US. The currently dormant caldera is approximately 3960 km2 -- 55 by 72 kilometers (34 by 44 miles)
Another comparable volcanic crater (caldera) on Earth is Lake Toba on Sumatra Island, Indonesia. It is approximately 3000 km2 -- 100 kilometres long and 30 kilometres wide, and up to 505 meters (1666 feet) deep.