(This is a small new section for the book beta. I will frequently post such sections as writing progress on the book continues. Comments welcome.)
There is an obvious tradeoff in structure size: small structures are more modular, allowing location decisions and upgrades to be made at a small granularity. Larger structures have more economies of scale in manufacturing and operations, and thus will be cheaper per resident. The choice of spar vs. flat seastead will have a strong impact – the spar is a large overhead cost which will be cheaper if shared, whereas the small concrete boxes to support flat structures are likely to be very cheap at any size.
Until we know the cost curve, we can’t determine a structure size, however there is one non-obvious consideration we’d like to mention. Seasteads are not merely structures to be engineered, they are homes to a community of people. And surprisingly, science has something to say about how big a community should be. Robin Dunbar hypothesized that part of the advantage of a big brain is that it allows you to have a larger social group. And he found evidence – within a species (such as primates, birds, reptiles, or fish) there is a very strong correlation between the logarithm of brain size and the logarithm of group size.
Based on human brain size, our optimal group should be about 150 people. And indeed, 100-150 is the size of hunter-gatherer tribes, military units, and the number of names in the average Rolodex of a modern city dweller. This is the largest group that is still small enough to keep track of reputation, favors granted and owed, and to all know each other. Groups at the high end of this scale require significant time devoted to social interaction, so 50-100 may be a more reasonable range. Some more recent evidence from social scientists who examine guilds in online role-playing games (MMORPGs) has confirmed this, see Dunbar & World of Warcraft and the related links at the bottom.
So while individual spar-seasteads may not be economically feasible, we should be careful not to let structures get too large. Our vision is of seastead cities as dynamically-shifting entities composed of modular parts, so it’s important for those parts to be small enough to hold a well-connected community, which can make good decisions about where it wants to fit in. If seasteads need to be larger than 150 people, we should consider subdividing them into 50-150 person sections.