One thing which frames my thinking on federation is "what is the minimum node size". On Mastodon the sweet spot is about neighborhood scale, i.e. 100-500 users. The sweet spot is different from the hard limits; you can run a single user Mastodon instance but the trade-offs make less sense.
Some people feel axiomatically that a minimum node size of one, i.e. fully distributed/self-hosted, is or ought to be the goal; I disagree.