I think they should make the next Fallout game (If they do make one) roughly the same size as Cyrodiil. It doesn't matter really where they make the game, because I thought New Vegas would be junk because it was in Vegas, but I love it. The only complaint is it's shameful size...

Or, they get a new engine and make it even bigger than Cyrodiil.

As the old saying goes - its not size that matters, its what you do with it.

If you had a map lets say, 3x the size of NV, but with the same level of content you really haven't gained anything, except a longer walk.

If you had a map again 3x NV and just filled it with minor locations you risk losing the main quest in the noise, and you increase development time

If you had a map again, 3x NV and increased the Main Quest accordingly then you've got a promise of an epic, but again increased development time and possibly risk diluting your good ideas a bit.

I'd just rather see a great game, of whatever size the devs think is appropriate.