How large do you want them?
The built-in icon images exist in sizes of 22x22, 32x32, 48x48 and 64x64, but Opus can scale them to other sizes (including larger) if you wish, or alternative icon sets can be used.
This icon set shows an example of how to add an alternative icon set which re-uses the build-in icons at different sizes: Internal, Small Icons for Context Menus -- It reduces the normal size, but the same thing can be used to increase it.
The .dis file can be renamed .zip to loo inside in, and you'll find an xml file within. (No icons, unlike most icon set downloads, since it reuses the built-in ones.)
DPI Aware Icon Sets in the manual explains how the scaling works based on the icon sizes and DPI scale factors or ranges defined in the XML.