IF this statement is correct: "We are 100% confident that Canon is going to announce something that will sit between the RF 100-500mm f/4.5-7.1L IS USM and the RF 100-300mm f/2.8L IS USM as far as price goes." I suspect that the front element size is going to be larger than 77 mm and smaller than 112 mm.
I would hypothesize a zoom lens with a 95 mm filter size because going larger than this size usually becomes more costly.
Assuming a 95 mm front filter size would mean that the long focal length of the lens would be either 400 mm f4.5 (likely the recently announced Sony), 500 mm f5.6 (like the Nikon PF lens), or 600 mm f6.3 (like the Sony lens). The only question that remains for me is what is the focal length on short end (likely 200 or 300 mm) and what is the weight.