Singapore has claimed that a two-tennis-court sized mindmap all about Singapore itself, is the world’s largest: http://blogist.wordpress.com/2007/11/17/worlds-largest-mind-map-unveiled/
There are hints of an even larger one (“well over the size of a football field”) here: http://pkab.wordpress.com/2007/11/13/capturing-the-second-life-worlds-information-in-one-giant-map/ but that’s in Second Life, so it’s virtual. I’d call that pushing the claim a bit, wouldn’t you?
Vic