@Thenomain
Oh, I get where you're coming from on the concept of scale. It takes some outside the box thinking and willingness to be expansionist in your design. The real world city I'm replacing with a fictional city for my WoD game, for example, I spent a lot of time looking at it, surrounding cities nearby (like LA and San Francisco) and determining what size would work, what would allow a good growth and what made SENSE in context.
I don't think it's undoable in the case of Fallout or any other genre, really. Some might be more funky than others (I've thought, many times before, about how to design the DC Beyond game I have in mind, but the scale ends up being a big issue because I can't reconcile things like Metropolis, Gotham and other cities... and I come from a background where games have entire Earth maps set up to account for movement and play), but you could come up with reasons and logic for it.
Or take some nibblet of info from the setting and go 'they mentioned this, said it was populous, but didn't show it... so let's use that'.