X and Y dimensions (in centimeters) for a landscape are calculated by multiplying the overall landscape resolution by the landscape scale. If the landscape actor is at default scale, which is 100x100x100, and your landscape resolution is 4096x4096, then it’s 409600cm x 409600 cm (or, roughly 4km x 4km).
Assuming a normalized height map (darkest pixel is black, brightest pixel is white), the Z-dimension of the landscape is always 512 meters (going from -256 meters to +256 meters) when the Z scale is 100.
Whether these dimensions actually match the heightmap you imported is a different question, and depends on the resolution of that heightmap (i.e. is one pixel on your heightmap one meter, or two meters, or five, or ten, or whatever. Depending on where you’re getting it and for what part of the world you may not be able to get better than ~30 meter per pixel height map data), and what the altitude range of the heightmap is meant to be.