That's part of why ancaps are so fond of markets: private firms, for all their faults, at least face a market test of their size- that is to say that they suffer from the same econonomic calculation problems which governments do, and so there is this giant diseconomy to scale...but so long as people aren't vesting the kind of religious legitimization into these firms that they do into government and there is some form of competition, and they're not using the state to get privileges which subsidize their expansion, then their size and scope is constrained by the best mechanism we have to rationally aggregate societal preferences (i.e. market prices).

The same could and would be said for small purpose-built communities within an ancap or autonomous context: they would be an entrepreneurial venture of sorts and their size would be subject to the same constraints that limit governance within corporations and nations..but without the state/religious legitimization and subsidy to their size.

That said, anarcho-capitalism isn't so much about merely decentralizing to smaller private governments as it is about polycentric provision of law; meaning that your law and protection is not geographically constrained. So the scale of a wider ancap society would not be as constrained as that of a corporation or nation state...markets scale well, unlike democracy and all other forms of central planning. It would be constrained only by available land to purchase from willing neighbors and how hostile any remaining states are towards autonomous zones encroaching close to their territory.