The Tsar Bomb
The largest bomb detonated in history was the 57,000 kiloton (57 Megaton, or 57 million tons of TNT explosive equivalent) Russian test in 1961. The fireball was 110 miles in diameter; the radius of the fire zone = about 64 miles. The total area of mass fire, or firestorm, would have been greater than 10,000 square miles if detonated over a large city or urban area.