Out of sheer boredom, I decided to calculate the total area of Blackreef and, as a bonus, count every eternalist and visionary. It was long and painful, especially counting every head on the island's four (technically five) main areas.
The area part was a little easier, though a little confusing. I first wanted to calculate the area of Updaam by measuring Colt's height using doors, which proved to be useless, as the doors in the game are either incredibly tall, or Colt is a 4-inch-tall gnome. So I used the game's markers to calculate Updaam's area, and then the entire island's area based on my calculation. Eventually I came to a conclusion, that Updaam is around 10.8 square kilometres big (4.17 square miles).
The population part was a lot harder, because I had to go through all the 4 areas (including Berezin's Manor in the morning) at almost all the four times of day. Keep in mind, my head count is far from precise and sometimes I had to make educated guesses because I simply didn't see all the eternalists. So eventually my chart looked like this:
Updaam had 49, 70, 62, and 94 people (275 in total)
Karl's Bay had 54, 45, and 39 (138)
Fristad Rock had 76, 45, and 60 (181)
While the Complex had 67, 37, 31, and 15 (I counted all the obviously dead eternalists too; 150)
I also counted all the visionaries, certain named eternalists, and also all the ones at Berezin's Manor
With all that, my total number is around 764, though, again, this number could be slightly higher or lower. I did that whole thing just to see if it makes sense for so many eternalists to be on the island of Blackreef, and, honestly, it mostly does, except for maybe the Complex in the morning, as I have no idea why there would be 67 eternalists in it, but oh well. It's also impossible that any of the eternalists in one area appear later on in another, because killing them does not change the number of enemies in any other area, at least I haven't noticed that.
If you want, you can do your own head-count. It sure is an interesting activity.