Going back to a picture that I've posted before:
The area inside the red outline is about 155 acres. The ponds are a bit more than 20 acres combined, so the park theoretically could be as large as slightly over 130 acres. There will also likely be a hotel that takes up some of this, meaning that the final size will probably be less than 120 acres.
Given the wording about being able to add "any world", I wouldn't be surprised if much of the northeast section (what I've labeled with a green 6) is reserved for future expansions, leading to an opening day park size of only 70-80 acres. Leaving wide open expansion space for additional worlds also probably means that there won't be many smaller attraction-sized expansion plots within the phase 1 worlds, which makes smaller lands and a smaller initial total park area more reasonable. However, the likely ~80 acres it opens with is nowhere near enough to be considered one of the largest they've ever built in terms of land area, so I'm not sure what that comment is supposed to mean.
Edit: I'm interpreting Alicia's statement as that the main priority is Comcast wanting to keep short term capital expenditures under control, and putting off the construction of the last couple of worlds for 5-10 years is a really effective way of doing so. It could be that the intent really is about preserving the ability to add a future IP, but that isn't the impression I'm getting, especially given the lack of planned water lines and driveways in the large area 6 on my map.