There is a workaround for now, of setting the max size for the IR, this way if your final render is larger than window resolution, it will still fit. Edit -> Preferences -> Corona, under Interactive Rendering, set Maximum Resolution to, say, 1200 for it to never be larger than 1200 wide.
(This is not to remove the request for auto rescaling, just to give you some way to work with large final render sizes for now).