Depends on the energy yield of the warhead.
A airburst 15 kiloton weapon (the yield of the Hiroshima bomb) will destroy or severely damage an area of about 10 square kilometers. The long-term radiation effects of such a weapon would be minimal - after all, Hiroshima has been a thriving city for decades with little or no adverse health effects.
An airburst 300 kiloton weapon (about the standard Western warhead yield) will destroy or severely damage an area of about 70 square kilometers. The total area affected by radioactive fallout from a single weapon could be as much as 10,000 square kilometers (in a ground burst), but this includes relatively low levels of radiation that would be mildly harmful over a long period - perhaps a tenth of that area would have dangerous levels of potential radiation exposure (and the actual dose for persons in that area would depend on exactly what kind of shelter they have, the terrain around them, and so on - not everyone would actually receive large doses of ionizing radiation).
An airburst 1.2 megaton weapon (the largest currently available in the West) would destroy or severely damage an area of about 200 square kilometers. The total area affected by radioactive fallout from a single weapon could be as much as 35,000 square kilometers (in a ground burst), and perhaps 1/8 of that area would face dangerous levels of potential radiation exposure.
An airburst 5 megaton weapon (the largest currently available in China - the large yield is needed to compensate for inferior accuracy) would destroy or severely damage an area of about 500 square kilometers. The total area affected by radioactive fallout from a single weapon could be as much as 75,000 square kilometers (in a ground burst), and perhaps 1/5 of that area would face dangerous levels of potential radiation exposure.