I'm not too big a fan of going full Points of Light myself. You'll end up with a lot of small isolated communities where safety isn't necessarily guaranteed, and there's not going to be a lot of diversity when everyone's all holed up against the bad guys. A lot of very big cities don't work in a D&D setting either. With all of those big cities throughout history, keep in mind many of them were capitals of major civilizations. Rome was only able to maintain its population on the back of constant grain shipments from North Africa, as the Empire declined the shipments fell off and the population plummeted. China maintained a high population because they developed agricultural advancements that could support those high levels, in particular a specific strain of rice that was able to feed large numbers of people IIRC. Europe by comparison lost population from the collapse of Western Rome, and then another significant population loss from the Black Death, and they didn't have China's centralized authority holding them together. As a result, big urban centers didn't start spreading in Europe again until the beginning of the modern era.
Generally the advice I've usually seen for D&D is that most of the big cities should have a population between 20-30,000 people and anything over 50,000 should be rare and significant communities.
The Realms are slowly recovering from the collapse of great empires in the past, primarily Netheril in the North. There are a few major areas of civilization. Waterdeep is the biggest city in the North but there's also places like Neverwinter and Silverymoon which are bastions of civilization. Further south, there's Baldur's Gate which is the biggest city in the region, though its importance has probably been boosted from the popularity of the Baldur's Gate games. Further south are Amn, Tethyr, and Calimshan, but they're south enough to not be threatened by the dangers of the North. In the center of Faerun is Cormyr which is another fairly important kingdom and they have a long running rivalry with the merchant kingdom of Sembia to the east. The Moonlands are dominated by Zhentil Keep, where the villainous Zhentarim threaten the Dalelands and Heartlands. Mulhorand and Unther are old ancient empires in the East and Thay is a nation of scheming evil wizards that broke away from them several centuries earlier. So there are some main powers in the Realms, but there's a good deal of empty space regardless of what the critics say.
The two starting points usually recommended for the Realms are the 1987 Grey Box for 1e and the 2001 hardcover for 3e. The Grey Box in many ways is FR's equivalent to the 1980 Greyhawk folio or 1983 box. The FRCS book compiles all the developments from 2e and presents a very comprehensive setting balancing the setting information with the early 3e rules.
