A Pelical mirror set up may work, at least for shorter focal length. For super-telephoto, It will lost some view at the top.Why would a pelical mirror need to be smaller, and why would the focal length matter?
It is the opposite. Since Pelical mirror does not move, it can be made bigger and be repositioned to be able to give us the additional 3.5mm area on the top and the bottom (May be??). A long lens actually need a bigger mirror due to the narrower object angle. You can try to draw it out yourself. or you can try to read some of the manual of film SLR in the 60's, they do mention the above effect. Now, DSLR never talk about this is due to the improvement of the mirror movement ( especiall, Nikon), the pivot of the mirror moves backward before swings up to allow a BIGGER mirror.