That nation can improve its environment to 1, ignoring GRL, with Borderwalls. The description for borderwalls says:
Border Walls - $60,000.00 - Decreases citizen count -2%, increases population happiness +2, Improves environment +1. (Does not affect Global Radiation levels)
So imho the best Environment a nation can have should be 1 + GRL (or 1 + GRL/2 in case the nation has RadiationCleanup).
The nation has 4,381.778 mile diameter.
3,407.207 in purchases, 724.883 in modifiers, 249.688 in growth.
No events. Federal Government, which should bring no land increase.
Only land modifiers are: Spices = +8%, Sugar = +5%, Wheat = +5%
Total of: 1.08*1.05*1.05 = 1.1907
3,407.207 in purchases * 1.1907 is = 4056,9613749 [649.7543749 in modifiers, not 724.883].
Including the natural growth of 249.688, that nation should have 4,306.6493749 miles of land.
It currently has 75.1286251 miles too much land, or one of the resources/gov brings a land bonus which is not listed.