Here is a size comparison of the recently announced Canon RF 200-800mm F6.3-9 IS USM with two other super-telephoto zooms from Sony and Nikon: Sony FE 200-600 F5.6-6.3 G OSS and Nikon Nikkor Z 180-600mm F5.6-6.3 VR. Real life scaled comparison images include the Canon both in extended and regular states. The size comparison is followed by a specs and feature comparison of these three giant lenses.
(click on the image for larger version)
Here is when the Canon lens is extended to 800mm. Sony and Nikon doesn't change length since they are internal zoom designs:
Canon RF 200-800mm F6.3-9 vs Sony FE 200-600 F5.6-6.3 G OSS vs Nikon Z 180-600mm F5.6-6.3 VR Feature Comparison
Check these detailed comparisons too: