What size should I set the icon to in order to match the built-in icons?
If I want to change the built-in icons, which file should I modify?
If you haven't already, take a look here: Icon Sets [Directory Opus Manual]
I personally would NOT recommend altering the built-in icons in their default set.
Large is 32px Small 22px I think.
This is useful for beginners Iconset Maker - #7 by galaxyhub