Why not use ZIP? Go in the Finder, select the file/folder, File menu > Compress "file/folder"
If you insist on using RAR, there is an option for that in Stuffit preferences (not Stuffit Expander, but the complete Stuffit package).
You can also use Keka, with the 7z option. 7z makes the smallest files, but might discard mac-specific metadata when compressing (e.g. spotlight comments).