Judging from my experience, you'd need one about the size of Ohio since they insist on not preparing enough food.
Seriously though, I've had much better luck making a bunch of decentralized canteens, even with much smaller prisons. I had a prison that got up over 100 and was still using one central canteen, but due to some poor design decisions I had to allot three hours for meals because it took up to two hours to walk there from the slave pits reformative workshop area.
In all my prisons (Alpha 28), the kitchen AI has proven it needs some work still. The cooks never, repeat NEVER, grab enough food from the deliveries area and consistently undershoot on the amount of meals. I'm willing to write that off to some subtle architecture flaws though.