Might as well add my findings here....
Southeast corner of mainland is approximately 12507.65, -15994.22 (couldn't get to the actual corner, waypoint teleport wouldn't go any further)
Northwest corner of mainland is approximately -12195.19, 8584.709 (overshot to compensate for previous)
That gives an approximate overall distance of 24500 each way. If that's in feet, then it's 4.6 miles each way or around 21.5 square miles for the mainland US/Mexico.
Tried with the other corners and similar numbers. Turns out the waypoint teleport wall is along the southern edge. Managed to get better measurments of the east and west coasts which were 12753.33 and -11712.2 respectively = 24465.53. Note that's at the water line. The top of the cliffs is set in further.
Top of cliffs (approx):
North = 8000
East = 12600
West = -11585
South = -16400 ???