How do I set the right margin?

I’m currently limited to about 50 characters per line. This is a problem in only one of my 2 current binders.

Make sure you have the ruler visible (Cmd-R), first. Then select all (Cmd-A) and drag the right-most blue arrow all the way to the right. It should snap back a bit when you do this, this is okay. Ordinarily Scrivener sets the margin width automatically depending on the size of your editing session. It sounds like for two of your documents, that got moved over at some point and once you do that, the automatic margin turns off.

Unless you mean something else by two of your binders having weird margins. The Binder is that thing on the left where you can select files—it doesn’t have text margins all by itself.