The blast radius of a nuclear bomb is variable. According to the National Terror Alert Center, the contributors to the blast radius of a nuclear bomb include the yield, fuel, weather, geography, whether the explosion was in the air or at the surface and even time of day.
The blast radius of the bomb dropped on Hiroshima was about 1.7 miles. The largest bomb as of August 2014, the 100 megaton Tsar Bomba, had a blast radius of 7.7 miles. The fireball resulting from the detonation of the Tsar Bomba is estimated at 1.8 miles, with a thermal radiation radius of 47.8 miles.