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.
Hmmm.... I'm not convinced. Wouldn't you get limb darkening unless the mirror reflected the full beam? Unfortunately I don't have any camera manuals from the 60's lying around Perhaps you can direct me to a more accessible source for information?
Sorry, I do not have one either. I try to search one for you in the web, no luck. This is a simple exercise to convince yourself:
Lay down a paper in landscape position, draw a 24mm line at the left end of the paper (The 24 mm line represent the height of the film or a FF sensor)
draw a perpendicular line at the middle of the 24mm line
draw a 45 degree line(23 mm long) from the top of the 24mm line, This line represnts the mirror.
draw one line to the top and one line to the bottom from the perpendicular line at 50mm from the 24mm line. this represents the "light cone" from a 50mm lens.
draw one line to the top and one line to the bottom from the perpendicular line at 200mm from the 24mm line. this represents the "light cone" from a 200mm lens.
Now look at how the two "light cones" intersect with the "mirror".
You will see that the 200mm "light cone" will hit a larger area of the mirror than the 50mm "light cone".
This proves that a longer lens needs a larger mirror. It will affect the viewing only, It will not affect the actual picture.( the mirror has swung out of the way)
You can go through the same exercise on the proposed 31.5mm square sensor also.