Found this on the r/HaloStory.
From the demo that was shown, and what can be viewed on the TacMap, pretty interesting to see the approximate size of the map. Horizonal: 15.4km and Vertical: 8.8km. Just note that the map is zoomed in, but this roughly gives an idea of how big, map wise Infinite will be.
More info can be found in the following link, such as images etc.