I am writing the section following a critical space battle where a rogue UNN Pearl of the Stars is taken down, and its CIC, reactor and, most importantly, its payload of 4 planet buster 15-gigaton interplanetary missiles, are secured.
The thermonuclear payloads are split into 5 warheads, weighing in at 3-gigatons each, for a total of 20 warheads needing to be pulled off their missiles. Think the UN missiles used against Eros in The Expanse for the kind of setting I am talking about.
Considering the kinds of nuclear weapons that have been designed and conceptualized (and declassified), what is the theoretical limiting factor for how small a gigaton-class nuclear warhead can be made.
Of course no one can put an exact number on it, but considering I don't need a scientifically accurate nuclear weapon that I expect someone somewhere to build someday, I only need to be in the right ballpark for it to be plausible to someone actually familiar with nuclear engineering. (It isn't going to fit in a briefcase.)
How small can one be made? The size of a large crate? A taker truck? 8 intermodal shipping containers? And more importantly, how much would it weigh? 500 kilos? 4 tonnes? It only matters because a few engineers need to lug it back onto their ship, and I need to know how hard of a time it will be for them.
I haven't locked this fact in, but I assume that the weapon would be designed to be as clean as reasonable considering the design constraints, possibly using antimatter as a catalyst. (but I don't understand enough about nuclear weapons to know if this is viable or will make it smaller)