Alton Towers actually isn't nearly as large as you'd think, if you look at it on Google Earth. A very broad-brush covering of everything that could be the theme park itself came out at
184 acres:
And even a broad-brush covering of the whole feasible resort area, including the hotels, car parks, field car parks and even some fields and houses that I'm not even 100% sure are part of Alton Towers came out at
501 acres:
I have absolutely no idea where Alton Towers' 900 acre figure comes from, or even the 500 acre figure I sometimes hear given that the second estimate covers large areas of land that I'm not even sure are part of Alton Towers...
As for the answer to the question; I'd always heard
Disney's Animal Kingdom and
Six Flags Great Adventure given as answers to this.
Animal Kingdom, when I made two polygons to broadly set out what appeared to be the full theme park area, added up to
301 acres:
And when I added an extra polygon for the car parks, the overall feasible area of Animal Kingdom added up to
386.6 acres:
In the case of Six Flags Great Adventure, a broad brush estimate of the theme park area itself, including a little bit of car park, comes to
201 acres:
And when adding the whole resort, including Hurricane Harbour, the Wild Safari (I wasn't sure if this was part of the theme park itself or not; the map implied that it wasn't) and the car parks, the total feasible area of Six Flags Great Adventure's whole resort comes to
808 acres:
So my findings would suggest that the largest in terms of the theme park alone is
Disney's Animal Kingdom at
301 acres, but the largest in terms of the whole resort is
Six Flags Great Adventure at
808 acres.
Unless, of course, there are larger parks that I'm missing...