The Just Scan feature is only available for certain inputs. The manual specifies this on Page 23:
Just Scan: Use the function to see the full image without any cutoff
when HDMI (720p/1080i/1080p), Component (1080i/1080p) or DTV
(1080i) signals are input.
Hope this helps.
I'm trying to set the picture size to just scan. It won't highlight and I get a cut off picture. Any help appreciated.