Leo, this is not good advice, as all other font sizes are OK, only the font size of the Input Edit control in the Create Folder dialog is too small. And what about making the dialog horizontally expandable? And have you compared the size of the dialog title and the font size of the Input Edit control on the screenshot? Obviously DOpus has a bug by not adjusting the font size of the Input Edit control together with the other font-sizes. I believe your concept of "standard font sizes" is wrong.
Suggestion: The DOpus Preferences should have a setting for Input Edit control font size!
What are your Windows DPI scaling settings? What's the monitor resolution and size?
Do you have any compatibility settings turned on for dopus.exe? Those can cause the wrong font sizes to be used. All compatibility settings should be turned off:
Other than that, Opus uses standard system fonts and sizes. Something looks wrong in your screenshot as your titlebar font is much larger than the font inside the dialog. Compatibility settings is one possible cause of that, but if it's not that, did you do something to change the titlebar font independently, or are you using any 3rd party tools to adjust the size of things outside of normal Windows settings?