- An area is one terrain block
- An area size of 128 means there's 128x128 units in each area
- According to the export guide, 1 unit is 1 meter
With an understanding of the above, you can come to the conclusion that each area in your world is 128x128 meters or 0.128x0.128 kilometers.
Assuming your world contains every area (i.e. no gaps, is a perfect rectangle) between (-12, 5) and (42, 26), your world stretches across 55x32 areas for a grand total of 1760 areas.
55 areas x 0.128 km = ~7.0 km
32 areas x 0.128 km = ~4.1 km
7.0 km * 4.1 km = 28.7 km²
Or, (1760 areas * 0.128 km) * 0.128 km = 28.8 km²
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Also I should note that scale is what you make it. You are really the one who determines the final scale. You could make your character have a scale of 0.0001f and fit your entire world onto 1 area if you wanted and still have the appearance of a world with 1760 areas, for instance.