There's no point retaliating once you're dead unless the enemy knows it's something you might do. You also can't make a plain A-bomb arbitrarily big as you need the fuel to be small enough to be subcritical until it's assembled, and simple enough to assemble that it spends so little time critical but not supercritical that a random decay doesn't cause a chain reaction to start before the mass is fully compressed. If it starts too early, there's enough energy to blow the bomb apart, which stops the reaction continuing. The more material you add, the more often random decays happen, and the likelier it becomes that the reaction starts prematurely. The theoretical limit is somewhere between 500kT and 1MT, which isn't very much for a city buster, especially if you've buried it. You'd have to use more than one, but a pure fission bomb is very senstive to nearby nuclear detonations, so only the first one would be likely to work.