How to fit an image to a plane maintaining the ratio of the image?
To have more control later on (scale and move the texture), I set the mapping to Flat and set Tiling off.
When I drag the material (with the image) on the plane (1000x1000) the image is place on the plane without taking the ratio into account.
I can do it manually by using Camara Map as projection and then eyeball the correct by moving the camera over (here) Z axis.
But when I then change the projection to Flat, the scaling is no longer correct.
Although the Coordinates - Scale of the texture tag stay the same.
Together with the help from the c4dlounge.eu forum the issues is solved.
The main issue was I did not read the manual good enough.
"Note that Size symbolizes the axis size and not the expansion of the object itself. For planar projections, the true dimension is the two/fold of the axis length."