For 1 texel per centimeter, a 128K*128K texture gives you 1.3 * 1.3 km, so id probably has about 1 texel per inch or so which is 3.2 * 3.2 km. Still not that big for wondering around in a wasteland using a car... though if they can make the levels non-square, they can get more out of the megatexture.