Go to Top View:
Middle click on one side of the Landscape and drag to the opposite side:
The measurement unit is in ‘cm’ (centimeters), so that’s the length of one side in centimeters. Multiply by 100 to convert to meters or 100,000 for kilometers then square (x2) the result to get the area in meters or kilometers squared, respectively.
You can make sure that this works by measuring the default cube, which is 1x1x1m:
Hope this helps, even if a bit late 