Offline game and big world : how to?

Hi everyone
I’m creating with some friends an offline game. Though, we are trying to create a huge world, and we’d like to know how to create a huge world. I mean, I don’t really think creating it in a single level file is a good idea (or is even possible) since it will take ages to load and will be very slow to edit also. So how should we create it ?
Another thing, does the Seamless Travel thing also concerns single player or not ?

I think you are looking for Level Streaming.