So what is the biggest possible nuke that can be made? MT of course. What effect could it possibly have?
You can keep chaining fusion secondaries together, but eventually the bomb blows apart before the Li-D can finish reacting. The theoretical maximum yield is probably not known with any degree of certainty, and if it is known the number's classified, but I've seen references in memoirs to somewhere around several hundred megatons as the theoretical maximum in a single device. Of course, you could also strap multiple devices together to make an even bigger bomb if you had some reason to. There've been occasional nutso proposals in the military-nuclear community for gigaton-range superbombs, none of which, as far as we know, got beyond idle speculation.
(Including one proposal for an Orion-drive-powered ICBM as a first-strike weapon, tipped with a 5,000-MT bomb. The backstory is the study writer actually wanted to build an interplanetary spaceship, and was using the ICBM idea as an excuse to play with the equations and get access to classified material on propulsion bomb unit data - apparently he arbitrarily decided the mass of the warhead would be exactly the same as the payload for a trip to Neptune.)
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