I saved a docx file as RTF. Then I imported it. For some reason the lines were short. It didn’t change the page setup as far as I could tell (which oddly starts with teen-tiny page margins). I tried using copy and paste, same problem. I had to copy from RTF to notepad, then copy that into Scrivener editor. And then reformat to double spacing and indenting first line.
Documents will import with the ruler settings from the original document, so if you need to fix it once it’s in Scrivener the easiest way is probably just to use Documents>Convert>Formatting to Default Text Style. This will preserve character formatting like bold, italic, etc. (which going via Notepad or using Paste and Match Style won’t do, since that strips all formatting) and will give you options for preserving other aspects but otherwise will bring the document into sync with with your current default formatting settings (which you can adjust in the Editor tab of Tools>Options).