I do agree with you that the zbench theme is narrow. Isn't necessarily bad, but for some stuff it may not be so good neither.
For my example, i had an issue where i had a page with tables so it couldn't fit in the whole page.
So i had to remove the sidebar with the widgets from that page alone.
Zwwoo sent me a no_sidebar file to put into the zbench folder. And on that page, just select that as the theme for that single page.
So that sorta fixed the problem for me. You can contact zwwooo by mail if you need that file.
Or like you said, maybe an option to change the zbench width in the zbench options would be nice as well.
PS: i strongly recommend you donate to zwwoo if you expect good support from him, which he already does for free. But i'm always confident going to him for solutions for his theme that i ended up donating myself :} was worth it.