the problem:
the RAW-file is a photo of 3:2, the target is to bring it to 16:9 because on this one pic i dont want to cut off anything, means i don’t want to crop.
Whatever i try with the resize module ends up in the given aspect ratio of the RAW-file.
If i understand RawPedia correctly, the ratio is automaticly adapted with the specify.
Is there a trick or function available to make it possible to change the ratio by resizing, which i overlooked?
To my knowledge you cannot do scaling using RawTherapee.
You can crop, but this will remove parts of the original 3:2 image or you can resize, but this will keep the ratio (be it 3:2 or any other you set with the crop tool).
You’ll need something like Krita or GIMP for this.
I wonder if the question was about letterboxing (adding matte borders) rather than scaling.
If that would be the question: Still not possible.
RawTherapee doesn’t have a border/framing functionality. You can be creative (Add a border to an image), but this will remove the outer parts of the image (border is not added, it is created from the edge inward).
@Jade_NL
thanks for your answer, i am happy not having overlooked a “simple” solution 
thats how i finally solved the “problem”
@afre good idea, which also would solve the problem, but my question indeed was to scale the pic.
thanks again!
Another possibility:
magick toes.png -resize 16:9 x.png
