Infranerd, that isn't their official website, but nonetheless I have seen them peddling 2,500 acres on their actual one. That probably just includes the residential developments minus the racing circuit, golf club, gardens etc. Jaypee's project page
however lists 5000 acres:
Overlaying the map on the actual site reveals that the Sports City "East" parcel of land is a conservative 4.5kms long and 1 kms wide. That comes out to be ~1111 acres, so you can gauge how big the main part of the city on the other side of the expressway is.