For GTK3 applications:
env GDK_DPI_SCALE=1.25 somegtkapp
For Qt5 applications:
env QT_SCALE_FACTOR=1.25 someqtapp
Replace 1.25 with your preferred scaling.
I believe there’s a different variable used for “fonts only”, however. (But the above should do what you’re looking for.)
I also believe you might introduce glitches if you veer away from 25% increments.
So these work best: .5, .75, 1, 1.25, 1.5, 1.75, 2, etc…
If you’re happy with it, make a custom shortcut or .desktop entry, and replace the Exec line.
So if it looks like this:
Exec=someqtapp
Change it to this:
Exec=env QT_SCALE_FACTOR=1.25 someqtapp
If you prefer the GUI route to make such edits, most menu launchers let you right-click → Edit Application → and change the “command”.