I've never played Minecraft, and basically the only knowledge I have of it thus far comes from seeing Game of Thrones' Westeros recreated in the video game world. I just got a new book called Block City: How to Build Incredible Worlds in Minecraft that I've flipped through, and it details a bunch of the game's biggest cities and worlds, showing how long they took to build, the number of buildings on those particular maps, and more. It also gives tips on how to create urban metropolises, fantasy kingdoms, futuristic zones, and historical realms, so maybe that'll get me into the game.
In the meantime, though, check out this cool 8-bit video from Wired, which talks about the global phenomenon and explains the absolutely ludicrous scope of what's possible in the world of Minecraft. I knew it was big, but I had no idea it was this massive.