"Biggest"
In file size? Or in square metres? Or cubic metres?
Minecraft can create an environment 8 times the surface area of earth, but it doesn't mean anything until you start talking numbers. In minecraft, 2 metres is probably not more than 50 pixels.
In the case of Minecraft, the surface area of the earth * 8
which is 510072000000*8 square metres
=4080576000000 square metres or 4,080,576,000 km2 (just over 4 billion km2)
This is pretty pointless without knowing the distance of one pixel, the the amount of data needed for one pixel, and since the maps are randomly generated, I think Minecraft is out of the question.
What we need is number of pixels, data used by each pixel, and how many pixels make up 1 metre (just to start with).
I did a calculation not so long ago, as follows:
I worked out that to store basic information about every cm2 of our planets surface
(basic information is colour(256 bit), x,y,z position, ocean toggle and surface type), The data file would be
21,513.72 Terabytes in size uncompressed. (21 million Megabytes)(21 thousand Gigabytes)
Most of this data is because of the colour depth. But as you can see if games are to start seriously boasting about having pre rendered planet size maps, we are gonna need bigger hard drives.
So far I think the game FUEL has struck me as having the largest map, and so it did strike the Guinness book of world records too. So I guess that's the answer for now.