Hi have a hi res monitor (3200x1800) on a 13.3 inch display , there seems to be a font size for everything expect the navigation tool bar where you save a favorite folder
Toolbar font sizes can be adjusted via Settings > Customize Toolbars.
From the description, I think the File Display toolbar is the one you want to adjust.
(Also, if you try the Opus 12 public beta, it should adapt the default font sizes for DPI automatically.)