Old Macdonald has the biggest greens collectively, but I don't think any one of them is a contender for the single biggest ... although I never heard how big #18 turned out to be. I think Old Macdonald has the 4 or 5 biggest greens I've ever built, all in one place, except maybe for the 13th green at Barnbougle.
If I recall correctly, the 18th at Indianwood (Old) is 22,000 or 24,000 sf, with a lot of humps and bumps in the middle. Once you get above 15,000 sf you are probably going to have to have a sprinkler head or two within the perimeter of the green ... that is the practical consideration which keeps most greens from going over a certain norm ... that, and mowing costs!