In bug #448761, I mentioned about the smart-sizing argument, which is great. However it has the caveat that the aspect ratio varies on window resizes. This option would be to reconnect to the server automatically when the window resizes to adjust the resolution (complete fit to the available space while keeping the aspect ratio).
This feature would be really nice!
Support for this has been implemented in krdc, but it needs a compatible server supporting the MS-RDPEDISP channel, eg. Windows >= 8.1.