Text Wrap and Text Size seem like they should be global to me. Probably Dark mode too, since that’s less about the style of individual documents and more about night/day or global preference.
In general, I think WriteRoom’s ‘brand’ is simplicity, so I’d lean towards global preferences for edge cases.
I haven’t developed a Mac app, so I’m not sure if this would work, but and ideal solution, for me: by default, all of these settings are global, but advanced users can define stylesheets that override the global variables. So changing a global variable affects all documents except those that have a custom stylesheet applied to it.