From what Wikipedia tells me, Boston is even bigger than Washington D.C, and if I recall correctly Bethesda started making Fallout 3 (and therefore Washington) right after they finished Oblivion. That's about 2 and a half years of a team of at least a few dozen people, working 9-5, every weekday.
Us modders? We have jobs that don't involve making cities bigger than D.C. We can only mod in our spare time, and worst yet, we don't get paid.
If I were you, i'd start off small. Make a settlement in a new worldspace built around The Institute or something. Because otherwise, you'll never finish this. I mean, look at those folks at Tamriel Rebuilt and Silgrad Tower. Sure, they make progress, but it'll be a loooong time before they're done.
(on a side-note, would the guy who's making that Tsaesci model over at the Akavir mod place, please just release it as a modder's resource already!?)