This depends on the technology. If you have self-replicating machines to do your building, then it doesn't matter what size thing you want - you set a machine off and tell it to come back once it has built what you want. Apart from resource depletion in an area (so just go choose somewhere unoccupied and otherwise useless), there is no real cost to this. If you achieve machines that can build other machines from raw materials unsupervised, then a stupidly-gigantic weapon - or 16 billion of them - is pretty much trivial except for the waiting time. Set and forget, come back when you want one to use. It would have basically a zero governance requirement.
In other, related, news, we don't really have a clue what post-scarcity looks like.
Certainly the "society with no reliance on coercive force" is attractive, although "virtually unlimited access to coercive force" might be a more realistic eventuality.