At first this question looked like "how far can the bomb fly?" But I have since realized that the question might be about a nuclear bomb's "blast radius," which is how far away the explosion reaches from the center. In this case, the main question is the amount of energy, or yield, in the bomb, which is measured in kilotons or megatons. Kilotons or megatons refers to the equivalent energy in thousands or millions of tons of TNT, and if you think about it, a one-ton TNT bomb can level a number of buildings. With a nuclear bomb, we're talking about thousands (kilotons) or millions (megatons) of times as much energy, at which point we're talking about leveling buildings in a few to several miles.
Here's a link that lets you model the blast radius of a nuclear bomb: