When I run Dolphin full screen at 3840x2160 with half the screen for the preview pane, the previews are too small to be usable. I’ve dug around in setting but can’t find any way to zoom in. Is there any way to show the full size document (PDFs in this case) or to at least zoom in?
Dolphin: 23.08.0
Plasma: 5.27.7
Framework: 5.109.0
Qt:5.15.10
At that screen resolution, the system is ideally run with full screen scaling, unless it’s like a huge 43" TV or something. Try going to System Settings > Display and Monitor and change the scale there to something above 100% until the sizes of objects on screen are comfortable. Don’t worry if this makes the screen blurry; reboot and that will go away.
After finding a scale that works for you, see if you still feel the need to increase the preview size in Dolphin beyond what’s currently possible.
I, too, would appreciate if the preview size would increase as I make the preview pane wider. 27" screen with 4k resolution here and no desire to scale the entire screen 
Use case: Renaming and filing lots of PDFs, it would be really cool to be able to read the first page of a PDF without having to open it in Okular.
Example:
(Screenshot edited for privacy.)
On a sidenote. A friend of mine uses krusader for this same thing. Dunno exactly what’s the name for it in English, but that fm has this “other” preview panel which seems to scale better.
This:
And it does the entire pdf ( or whatever)
Thanks a million!! This is so useful. Incidentally I even had krusader already installed on my system, never really knew how to make use of it, but now I do 
