How big does a large balloon have to be to contain a nuke payload/emp/etc. that can do significant damage because it's assumed to be just taking pictures? Would

How big does a large balloon have to be to contain a nuke payload/emp/etc. that can do significant damage because it's assumed to be just taking pictures? Would we be able to respond quickly enough if something was just let go from that balloon to detonante above an interesting target? From a tactical point of view this reminds me of a movie where an infected animal is walked past guards, penetrating deep into an enemy camp. Red Cliffs maybe? I see from another post that this is "the size of three school buses" - Seems the right size to carry a necessarily heavy payload? A spy balloon doesn't need to be massive to have good fidelity with modern image capture elements and would be more effective if not easily seen from the ground. Also could be 1 of n.