Hello @j_vogel,
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The parameter DOCUMENT_DOCUNIT in a document is just something that is added to things like the coordinate manager, to set the unit and a multiplier for the numbers shown there. It will not scale the actual geometries (as it otherwise would be destructive) and therefore does not affect the view port.
To scale all elements in the scene, you must scale all elements 🙂 . You can use the command which we implemented for that, what is talked about above and which can be invoked with the button in the document scale settings. But you then have to use its UI, which is I guess not what you want. So, you would have write something like that yourself. But there is currently no premade scene traversal in Python which can be a bit daunting for beginners (will change with the next major release). Carrying out the scaling itself is not that hard, although this complexity can also vary based on the content. Since I know you have beta access: You can also pick the latest beta build, the traversal stuff is already in there.
Cheers,
Ferdinand
edit: you should of course use the local and not global matrix, as you otherwise will scale things multiple times.