Terrain size is defined by heightmap grid resolution multiplied by unitsPerPixel property in terrain.i3d file. unitsPerPixel can be edited from GE GUI as well.
unitsPerPixel is a one heightmap grid point in 3D. Smaller it is more detailed and smaller the terrain will be and larger it is more larger the terrain will be. unitsPerPixel is meters.
mapDE and mapUS both use unitsPerPixel="2" and with 1024 x 1024 heightmap grid resolution they become 2km x 2km terrains. This means one heightmap grid point covers 2 meters in-game.
PMC Grande Gardens 16km is 16.3km size terrain. Its heightmap image resolution is 4096 x 4096 (in GE it turns into 4097 x 4097) and PMC_Grande_Gardens_16km.i3d lists unitsPerPixel="4" which means one heightmap grid point covers 4 meters in-game. PMC North Dakota Wahpeton 16km and PMC Texas Hereford 16km uses the same specifications.
PMC Eternal Sugar Beet Damnation 32km is 32km terrain which uses 4096 x 4096 grid heightmap and unitsPerPixel="8".
Decimal values work as well, for example unitsPerPixel="3.5" works okay. Its recommended to keep these as .5 or .25 numbers instead of going wild with .12345678 or similar.
You can also use smaller unitsPerPixel for example 0.5 and with 512 x 512 grid heightmap your terrain size would be only 0.2km. In our tests unitsPerPixel 1 with 1024 x 1024 heightmap produces 1km terrain which works OK.