I do struggle with the first line i.e. that need for protection correlates with size

That's what experience shows: in a community of only very few people, you can easily deal with all of them individually, and can often resolve conflicts before they escalate.

Have you ever seen a three-people community plagued by a troll?

I think actually that the smaller a community, with the individual being the smallest case, the more protection it needs from bigger groups or these days those claiming to represent the group by a faked mandate.

The smaller a community is, the less attention it draws to it, so it doesn't need much protection either. Also I don't see the faked mandate as the main danger of communities, rather it's trolls sucking up their resources and poisoning the atmosphere.

I think actually that the smaller a community, with the individual being the smallest case

An individual being a community? That seems like a weird notion.