This panel is unconfigurable and contains only preview
To me dolphin without panel on right side looks unusual and unfamiliar. Due to that I made patch returning it.
And that panel may be resized, having preview of bigger size than previews of files in the central area. Of course, central area can have files resized too to increase preview, but then the screen would be able to hold much lesser amount of files.
And like before, this panel may be hidden to not take screen space if it's desired.