I have no idea how far 20k words go...
I did a quick, hack job count with the Sail Away Sweet Sister PDF, and it came in at around 29k.
You made me curious, so I did the same for some of the 4th Edition location books. Here are my results (keeping in mind that these are counts of all text, including table of contents, afterwords, etc):
Corporate Enclaves: ~115k words, 138 pages (~833 words per page)
Feral Cities: ~115k words, 138 pages (~833 words per page)
Montreal 2074: ~17k words, 21 pages (~810 words per page)
Runner Havens: ~117k words, 146 pages (~801 words per page)
Seattle 2072: ~169k words, 202 pages (~836 words per page)
The Land of Promise: ~20k words, 24 pages (~833 words per page)
The Rotten Apple: Manhattan: ~24k words, 32 pages (~750 words per page)
The above certainly seems relatively consistent, so I'd expect a 20k word book to be around 24-30 pages or so.
That being said, the information contained in The Land of Promise is really good and it's at the same 20k word count that we're discussing here. To my mind, then, even at a relatively low word and page count compared to the behemoth that is the Seattle sourcebook, I think a 20k word book would cover Aztlan in an interesting way; the level of detail would be far less than how Seattle is covered, obviously, but I think it'd be an interesting book nonetheless.
For a more in-depth look at South America, though, I'd say something like the Corporate Enclaves and Feral Cities books would be more applicable; 120+ pages with 100k+ words. These are all quesstimates from my end, though, and as previously mentioned I'd be happy to see more material on everyone's "love to hate" mega, and Aztlan in general
EDIT:
While I knew about Gagarin station being a potential hot.-spot for CFD, I am thrilled that Run Faster flat out confirms it. From the Space Runs section on page 36:
"This is an especially good opportunity to have some CFD fun on Mars or one of the other space habitats (p. 166, Run & Gun) that have become a haven and breeding ground for the virus and the resulting head cases."
I can't wait for more info on this, and I would personally add a request for a space themed PDF, perhaps in the 20k word format discussed so far. Pretty please with space ships on top?