There is no preset world size. It's more based on limitations in disk size and even then it's hard to say. It only generates and stores the parts of the world that you explore... so if you walk in a straight line you can generate a really long and skinny world but it will basically go on until you fill the whole hard drive.
Now, there are some practical limits in the released version that set some virtual limits.
First, there is a sort of bug in the world fractal that causes the land to get really dramatic and crazy when you get down to coordinates in the -12,000 or so. The world gets really spiky. Eventually, at around -16000 the client will crash because one of these towers generates too tall and there is a bug where I don't clamp the value properly.
I think that limit is even in the new engine.
The other practical limit is in the player movement physics, if coordinates get really high then movement starts to feel strange. We've teleported up to 100,000 x 100,000 and started to feel like we were drifting a little as we walked. When we teleported up to 1,000,000 x 1,000,000 we couldn't even walk... just look around.
That bug is not in the new engine because movement is handled with a higher precision physics loop that can deal with the larger coordinates.
The amount of disk space that a particular section of the world uses will vary by a lot... so I can't even define fixed limits based on hard disk size. Also, the compression may get better in the future allowing for even more efficient world storage.
Sorry that I couldn't directly answer your question... but hopefully you see why now.
