I thought that if I specified the correct display size through an nf file the display would be shrunk to fit the screen, but this is not the case, the display image is still larger than the screen. It therefore seems to default to 65 inch.
I note that the EDID for the TV does not specify the display size. The system information for the screen reports it as a 65 inch Panasonic TV instead of a 50 inch. The image on the screen is larger then the screen on all sides, cutting off the edges.
It is connected to a 50 inch Panasonic Plasma TV (TH-50PX600A) via HDMI. I have Ubuntu (16.04 LTS) installed on a Gigabyte Brix (GB-BACE-3150).