When reading resolution of hightmap it is max 512x512 although I have imported a bigger one

Hello guys, I am trying to read data from the Landscape’s heightmap

UTexture2D* heightMap = landscape->LandscapeComponents[0]->GetHeightmap();
//texture resolution
int sizeX = heightMap->GetSizeX();
int sizeY = heightMap->GetSizeY();

But the GetSize functions return sizes of 2^n and that only to a Maximum of 512x512, altough the heightmap that I generated in the editor is much larger.

So for a hightmap with resolution 883x946 I get GetSize() = 512 and GetSizeY()=512. (512x512 is the size of MipMap 0)

Any idea how I can access the whole heightmap?

Thank you so much!!