How do I force a 16x16 icon size for toolbar icons in GTK 3?
Solution 1:
You can try this by creating/modifying ~/.config/gtk-3.0/settings.ini:
[Settings]
gtk-icon-sizes = panel-menu=16,16:panel=16,16:gtk-menu=16,16:gtk-large-toolbar=16,16:gtk-small-toolbar=16,16:gtk-button=16,16
However, your theme may override this in its theme file.