I had tried 2 as the input , and thought the problem persisted, but just tried it again, and indeed, the preview window is now fully used. Not sure why I missed it the first time.
I have Win 10 set to 125% globally. Works fine for all the apps I use .
Using 2 as the scale , really does make the fonts TOO big, which is why I used a lower value. The fractional DOES work on the fonts and interface, but not the Preview