Where is the biggest theatre organ in the world and what was it built for?

Bit of a clue, it's not in the surroundings you would expect!

Is it what you are looking for Jenny?


Convention Hall Atlantic City NJ .


Source

I have the Wanamaker department store organ in Philadelphia as the largest but there seems to be some controvesy about the answer to this!
The Atlantic city organ is the world's largest, but is not complete and not all working. The Wanamaker organ in Philadelphia is the worlds largest complete organ.

Bonus, what was the Wanamaker organ built for?
http://www.nightscribe.com/Music/pipe_organs.htm

In 1909, Philadelphia merchant-prince John Wanamaker bought the instrument for his new Philadelphia emporium. Thirteen freight cars were required to ship the entire organ from St. Louis, and installation took two years. The Grand Organ was first heard in the store's seven-story atrium on June 22, 1911, at the exact moment when England's King George V was crowned. Later that year, it was featured when President William Howard Taft dedicated the store.

source

Yes!
Built by the Los Angeles Art Organ company for the 1904 St. Louis World's Fair, the Wanamaker Organ was desiged by renowned organ architect George Ashdown Ausdley, author of "The Art of Organ Building". This heroic instrument had more than 10,000 pipes and its construction was on such a lavish scale that costs soared to $105,000, bankrupting the builder.

http://theatreorgans.com/pa/philly/WANN/index.HTM

Great question and answer! Thanks for the links; they were very interesting and informative.