The math is wrong for this.
27 grid units wide
27 grid units high
729 grid units in total
Your calculation was 145,800 feet, and that's how tall or wide the world is, not the total world size.
145,800 x 145,800 feet = 21,257,640,000 square feet or 762.5 square miles
To put things in comparison, Just Cause 3 has a 1000 square kilometer map or 386 square miles.
Looking at the map, that doesn't look right from a scale perspective. Maybe each grid unit is 100 feet by 100 feet?