What's the world size in Survival?

Hi,

Now that I can save my home, I'm going a few thousand X and Z each day to get to new lands, then saving home.

I've seen wood and stone villages, and some evidence of other players constructions!

But what is the world size of Survival?

I've seen 10,000 x 10,000 touted in many other multiplayer servers, but I dunno about MCCentral.

Also, are there underwater constructions or cities in Survival?

Regards,

Ambi