Heya =) Welcome to the community!
Well, I can't speak for big feet, I have the opposite problem, but I am very aware of knee problems. Didn't get mine surgically repaired and had the long process of it healing on its own, so on occasion when the weather starts doing something funny or the humidity gets too high, or if I over use it I start to feel it.
That being said, swimming in my monofin has not been an issue one bit. I use a Finis Aquarius (Not sure I would recommend that fin though), and after 1-2 hours of swimming laps with it I'm not having any issues with my knee. Your upper body and core should be taking most of the brunt from a dolphin kick
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