You need to tell Opus to save the new widths after changing them. In Opus 12 (currently available as a public beta) you can create a button or hotkey directly on the toolbar to do this with one click. With Opus 11, it needs to be done via the Folder Options dialog (just open it, then click save; no need to edit anything inside of it).
I am still not sure what you are trying to do. Are we talking about double-clicking on archives, right-clicking them, or something else?
See the FAQs: Why can't I edit my posts? (also Ask one question per thread just above that one ).