The list of participating subreddits[0] includes ~3/4ths of the big subreddits. 7 of the 10 subs with over 30 million members[1], 81% with 20-30 million, 77% with 10-20 million, 80% with 5-10 million, and 71% with 1-5 million

I don't think the blackout will be limited by its scope so much as by its duration

[0]https://www.reddit.com/r/ModCoord/comments/1401qw5/incomplet... [1]https://www.reddit.com/best/communities/1/