I think the biggest O gauge one is the one in Paterson,NJ. The largest HO is in Hamburg,Germany.
That would be the NJ Hi-Railers! http://www.njhirailers.com/
We're in the process of documenting the track plan in RR-Track. What a job! We've been working on it for several months now and it'll probably take a good part of this year to complete. Perhaps then we'll have an objective measure of it being the largest O-Gauge layout in the world. I'm willing to bet it will be the largest plan ever developed in RR-Track. Hope it can handle it.
I'm thinking of starting an over/under as to how many scale miles of track are in the layout! Winner gets the pride and prestige of being right.