The Commonwealth/Boston Wasteland map is going to be a incredibly denser than either F3, FONV, Skyrim, or Witcher 3.
DC was nearly totally destroyed by nuclear bombs and has yet to recover.
Las Vegas is mostly surrounded by desert and wilderness (Nevada ranks 42 in population density).
Skyrim was mostly wilderness and the Witcher 3 consist of medieval country side, wilderness, a few towns, and only one real city.
Per Wiki:
As of 2014, Massachusetts was estimated to be the third most densely populated U.S. state, with 839.4 people per square mile.
As of 2013, Nevada ranks 42 in population density and has 24.8 people per square mile
In 2010 Boston was estimated to have 617,594 residents (a density of 12,200 persons/sq mile).
Just for comparison, in 2010 Las Vegas had nearly the same population but had a population density of 4,222.5/sq mi or one third of Boston's.
And unlike Washington DC, it was mostly missed by the IBCMs.
For the last two, Medieval France tops the list, with a 14th-century density upwards of 100 people/sq. mile thanks to a lot of arable land.
Skyrim and the lands of Witcher 3 would only have been a fraction of that density.
So I expect the Fallout map to be about the size of the Skyrim map, but to be incredibly denser just because even if you leave out the 600 thousand Bostonians, the surrounding area had about ten to twenty times the population of any of the other maps.
Plus there is the increased verticallity thanks to the access to the power armor jumpjets and an oncall Vertibird.