File and folder icons should come from the system and use the system size.
But I am guessing you mean the toolbar icons. In Opus 11, toolbar icons can be swapped for a larger icon set. (One example "XL" set / another example.)
One thing you can't change are some of the icons used in dialogs (e.g. parts of Preferences) and the status bar format lock and things like that which aren't on toolbars or using the toolbar icon set.
In Opus 12, high DPI icons and (when it makes sense) icon DPI scaling are built into the program so everything is automatic, including using the same configuration across multiple DPIs. (Less obvious things like stored window positions are also handled in a way which considers the DPI of both when they were saved and when they are loaded.)
The Opus 12 betas are very stable now so I would recommend switching to them if you're using high DPI and want the easiest path, but Opus 11 is livable in high DPI with a little configuration.