Jacob 2

Jacob 2:2 I come up to the temple this day that I might declare unto you the word of God. Do you suppose Jacob called an assembly of sorts?  You get the distinct impression that he was speaking to all the Nephites.  The culture must have been incredibly close-knit at this point in time, to bring even the sinners who needed to hear his message to the temple.  If they were there, their presence might give us insight into the size of the group--or rather, insight into it's approximate maximum.  A scientist named Robin Dunbar did some research on primates and came up with what is now known as Dunbar's Number.

Dunbar's number is a theoretical cognitive limit to the number of people with whom one can maintain stable social relationships. He suggests that the group size is tied to the size of some part of your brain (feel free to click the link to read the actual article).  His number is only about 150 people.  There are a few other studies that estimate the number would be higher, with the largest being around 300 people.  The point of knowing this is that before you cross the line, there is some pretty intense social pressure to conform to the norms of the group, and once you cross the line, you get enough people behaving out of sync to create a sub-group, ergo, you might appropriately expect to have absenteeism from a polygamy speech by the prophet at the temple.

Now, some people might argue that the fact that the polygamy was happening is evidence of the society breaking into sub-groups already: those that support Jacob's teachings, and those that did not. I think they would be right.  Somehow, in that society, it would appear, it was less of a social stigma to pick up another wife and justify it by the written behavior code (the brass plates' account of David and Solomon) than it was to skip the prophet's speech at the temple.

I think, though, that we can use the attendance at the temple and the polygamy to pin the group size somewhere between 150 and 300 people.  Large enough to start to show fractures, but not large enough for an actual schism.

Of course, this is all speculation based on one interesting theoretical number derived from a study of monkeys.  There could well have been thousands of them and he might talk like that even though the sinners were absent.