Canon appears to have filed a patent application on specifically a 800mm F5.6 lens.
The patent attempts to help reduce weight, size while maintaining a high degree of optical performance and low chromatic aberrations. There are two embodiments, both are 800mm lenses.
Focal distance 780.00
F number 5.80
a half field angle (degree) -- 1.59
Image height 21.64 whole length of the lens 485.85
Japan Patent Application 2018-087965