:eek: So George lied? They really could not have destroyed Alderaan.
Oh no, a powerful enough* beam would do it. In fact, given how fast Alderaan exploded, the beam must have been powerful enough to destroy not just a terrestrial planet but a Jupiter-sized gas giant.
*like, the output of the sun for a couple of hours
If the question includes only fission bombs, there is an upper range (cite Richard Rhodes, The Making Of The Atomic Bomb, page unknown) because chain reaction- the runaway production of neutrons- propagates faster than the expansion of the medium (the blast wave). You can pile more and more uranium or plutonium in one place, but beyond a certain amount the blast wave outpaces the fission reaction and blows the remaining fissile material away from the reaction.
If the question also includes fusion bombs, then there is no upper limit. The fusion reaction propagates as fast or faster than the blast wave. You can keep piling more and more material (deutrium, aka heavy water) in one place and you will get a larger and larger explosion.
Yes, there is an upper limit to the size of a bomb that can be carried by sea or land or air, but the question isn’t about the largest movable bomb. Yes, beyond around 100mt any extra explosive power will tend to lift a chunk of atmosphere into space instead of increasing the ground-level blast radius, but the question isn’t about the largest effective bomb that could be built or the maximum blast radius.
The answer would appear to be that, given enough manpower, technical skill, resources, and material, there is no upper limit to the explosive power of a nuclear bomb.
Please don’t talk about The National Ignition Facility in Livermore, California. I live near there, upon the Hayward Fault. I have enough to worry about. Thanks. 
:dubious:
Hmm, so the plot of “Crack in the World” (1965) was a just a flight of fancy?
Let it not be said the Hollywood would let scientific detail stand in the way of good disaster movie.