The folks at Dexerto managed to preserve the post. According to the original poster, the map in GTA VI is "huge" and covers to major cities, those being Carcer City and Vice City. Carcer City is based on Boston, and Vice City is based on Miami.
"The map is huge, like stupendously huge, hence the game's emphasis on air travel. It makes GTA 5 look like a schoolyard playground. There is an array of atmospheric effects that bring out the life in everything. Light pollution, hurricane winds, morning fog, etc. I've never seen an open world game this realistic before," the leaker wrote.
Assuming all that is accurate, it would be a good opportunity to inject real-time ray tracing into the mix, though there is no mention of that. Instead, the focus is on the size of the map. The leaker says it's so big that it's "a little awkward traveling from Boston to Miami," in part because there are no other major cities in between them, in the game. Expect a lot of air travel, if that's the case.
The leaker also says GTA VI will again put players in control of multiple characters, just like in GTA V. One of the protagonists is said to be "an undercover cop or something," and other might possibly be a sibling. In addition, the leaker says Rockstar Games "complete revamped" the mission design.
"You can permanently abandon some missions or take different routes that sprout different consequences," the leaker says.