Those days, I'm considering how big the new zone should be. And the option I like most is creating a really huge map. But players won't have to explore all. I mean, the quests will take place in a small part of the whole map. The rest of the will be there just for people who want to explore.
Doing that, I want to give players the feeling of being in a huge world. WoW players know what I mean: you don't have to explore all Azeroth, but if you like it, you can do it.
GTA:San Andreas is another example. The game takes place in 3 main cities, but you can find small side quests in small towns lost in the mountains.
But before doing it, I want to have your opinions about that.