Now it is hard to determine the size of middle earth, but I will be using information from the movie for a rough estimate as of the distance between Edoras and Minas Tirith.
In the movie, Gandolf said it would take 3 days to get there. And Shadowfax is considered the top tier of all horses, so were will be using the top running speed of any horse which is 24.5872 m/s.
So plugging in the information with 3 days = 259200 seconds * 24.5872 m/s = 6373000 meters (roughly)
Here is the image:
So the distance with pixels is 422 px = 6373000 meters
Length = 2455 px = 37075164.5 meters
Width = 1677 px = 25325886.3 meters
So the Area is = 9.39e+14 m^2