By "the physical size of the screen is wrong," you could mean two things:
- The physical size is detected incorrectly: You can override this with the DisplaySize option in the Monitor section of xorg.conf.
- The physical size is different for each RandR mode: This is intended behavior to keep the DPI constant at the different modes. See the ConstantDPI option in http://us.download.nvidia.com/XFree8...ppendix-d.html
Hope that helps!