Make sure “Enable HiDPI support” and “Enable fractional scale factor” are enabled in Krita’s settings under Window (near the “Use custom interface font” setting). It sounds like you might’ve been looking at Window’s high DPI settings found in the application properties under Compatibility, which you probably won’t need to mess with.
To change the scale factor you could run Krita with the environment variable QT_SCALE_FACTOR set, it will be multiplied by the global scale factor.
So for instance, saving this as a .bat (Batch script) file
set QT_SCALE_FACTOR=1.2
start /d "C:\Program Files\Krita (x64)\bin" krita.exe
and running it would (after showing Command Prompt briefly) start Krita at 1.5x scale, if your global scale is 1.25 and your Krita is installed to that path.
If you then wanted to add it to the Start Menu, you could create a shortcut to the batch file, put it in %APPDATA%\Microsoft\Windows\Start Menu\Programs\, and use change icon to get the icon from krita.exe. Then you can run it pretty much the same as regular Krita’s shortcut.