Doesn't flare usually cause a lowering of contrast across the entire image circle? Thus wouldn't the size of the hood required depend on the size of the lens opening and nothing else? I ask this not rhetorically, but because I'm not sure.

It seems to me like a lens hood designed for a 75mm might still let some flare in on the 50.
[edit] Ah, a later response showed where I wasn't thinking right. A 75mm hood would actually cover the element more, which would be fine because of the reduced image circle. It makes perfect sense to me now. Disregard my earlier thoughts.