I'm all down with HUGE maps and explorable areas and all, but I think the key word is 'moderation'. I'm going through Pokemon Pearl again (this with Chimchar!) and one of the things that really griped me the last time I played, is there's a point where you have to hunt down some rare and (very friggin) elusive pokemon. It spawns randomly in a spot and shows up on your map, but instead of waiting there for you to come get it, every time you enter a town or travel through a different named area, it'll move. Sometimes a little, sometimes a lot. And if you try and take a short cut by using 'Fly' to insta-travel to the town next to it's current spawn, it randomly spawns on some other spot way the hell away from you.
So you have to go across the map, dealing with random encounters of enemies 1/5th your level (sometimes 1/20th) just to track this thing down. And once you get to it, you only get one chance to attack it, or it zips off to some other place across the map. So you better be faster then it, and even then, you have to face it multiple times. So pack a lunch, this could take a while.
What does that have to do with scale of explorable areas? Being forced to travel large distances, eating up time where I could be making progress in the game, is not 'fun' to me. Oh sure, it was interesting. The first time. But after that it gets boring.
So sure, make it big and expansive. Just don't make me spend more time picking my way to fun, then having the fun.