As far as I know, its not the biggest MR railroad in the world anymore (since 05).
I respect the commitment of the guy who build it, though.
This one is the biggest:
http://www.youtube.com/watch?v=ACkmg3Y64_s
Personally, I am not a fan of huge layouts that live from effects and show, but they attract the average non-railroad inclined people, so its good that they excist.
Northlandz is probably the largest MR thats been build by a single person.
The only thing that I know of that came close to this was a 1:160 exact (!) scale replica of the central station of Stuttgart.
While I searched for the link to show it to you, I´ve learned that Mr. Wolfgang Frey, the builder of that 750m² layout, died in 2012 and his website is not available anymore.
Very sad, he was a nice and forthcoming person that I had the pleasure to exchange some emails with.
http://youtu.be/lrZmEzcRaaA?t=9m15s Here is a small glimpse of it. He scratchbuild 100s of buildings and every turnout that was on the real thing was there.
As well as building this, he build a replica of the signal tower as well on 90m².
What a loss for the community.
Anyway, a club took some picture which you can find here:
http://www.modellbahn-dortmund.de/events/events_2011.html