Thats only an estimation, not something really accuarate. Also the world map shown in the tech demo videos suggest something way bigger.
Isn't that the idea? It's an estimate, I'm not trying to get an exact number here, one of my numbers can take the size of the map from under 400 sq km to over 1000 so it's just for discussion...
I've done other estimates in the past. If it takes roughly 1 hour to cross the Duscae region as was stated before, assuming Noctis travels at a pace of 3 miles (4.83 kilometers) per hour, accounting for distance traveled diagonal across the region and other obstacles 2.69 sq km actually sounds about right for the region as moving diagonally across the space would be traveling within roughly 2.3 sq km region of space which lines up close to the numbers I estimated here, off only by 690 square meters.
Besides, the 2.69 square kilometers is fairly accurate given that it was taken directly from the sources in the demonstration today.
We don't know how big the region is, we'll just have to wait and see, but it's still nice to speculate, isn't it?