A few years ago, Six Flags Great Adventure claimed to be the largest theme park by aera, when it changed the separate safari park so it became part of their regular park line up. I checked this just now and it seems it no longer is according to the SFGAd Wiki page:
On August 30, 2012, Six Flags combined its 160-acre (65 ha) Great Adventure Park with its 350-acre (140 ha) Wild Safari animal park to form the 510-acre (210 ha) Great Adventure & Safari park, making it the second-largest theme park in the world, after Disney's Animal Kingdom.
If Wiki is a trusted source, the answer is Animal Kingdom. I couldn't be arsed to find out how big DAM is.