To partially answer my own question.
The Arty S7 and Arty A7 reference an inner diameter of either 2.1 mm or 2.5 mm, but the Arty Z7 only references an inner diameter of 2.1 mm. Is that so about only 2.1 mm for the Arty Z7?
I haven't found yet the outer diameter, but am assuming for the moment 5.5 mm.
So I guess if I want a single power connector common for all three boards I should get a 5.5 mm / 2.1 mm power connector like this: https://www.amazon.com/dp/B071HB1589? If all three boards were to take either 2.1 mm or 2.5 mm, are there any downsides going with 2.5 mm over 2.1 mm, or vice versa?
P.S. I am not affiliated in any way with PWR+, I just found that brand first on Amazon. There are probably cheaper and/or better power adapter alternatives.