How do get the game to work as 1600x900 for example

i've set it to that in the DirectX config but that seems to be more for the classic version.

What you expect from that? What you mean from that? Game SHOULD stay as 4:3 window and IMPROVE IMAGE QUALITY is NOT POSSIBLE.

You want remove black bars? You cant. You can stretch image - but that reduce quality (blured) and broke proportions.

That is not a settings from game options. This is how you set up your video cards and display settings. Resolution is irrelevant to this.
Again. it if your point is about black bars.

Obviously i'm talking about the enhanced version, As there is no black bars with the classic one.

So it is about black bars.
You wrongly set you video card and display OSD menu (some monitors have option about scaling proportions). because Classic is 4:3 black bars and same for enchanted.

In enchanced right now with current driver settings you can stretch image too. There is keep aspect ratio option in GOG DXCFG - turn off.
But that stretch screnn without proportions and blured image. You should set keep aspect ratio everywhere where is possible.