The more I read about it the more I want to get "0mm x 0mm" to be set to the actual physical size of the TV.
Apparently it calculates the DPI of the screen from this and can calculate a non-1:1 pair of DPIs.
If the DPI is no 96x96, say 80x96 then the pixels are not square but wide!
Whether this will actually DO what I want it to do is debatable buth the math is sound!
Sadly the only way I can see to set the DisplaySize is by writing an xorg.conf.
As the xorg.conf file is redundant because all the values are now inferred automatically, is it possible export all those auto values to an xorg.conf file?