Going over the old Forgotten Realma Atlas from 1990 for fun, and I estimate that (give or take) the area covered in detail in this new book ia about 9,240,000 square miles or around about the size of North America, while the area to the South of Faerûn is about 6,440,000 square miles, approximately the size of South America (about 10% just being the band of the Shaar along the middle bordering the Nprthern area).
You could basically do another book of a similar scale just about the Shining South (the 3.5 Shining South is the same length, to be fair). And going through the Grey Box Compendium...not only does the map cut off around the same spot, there is no info on the Shining South to be found there. "The South" is given as another name for the Old Empires region!
