I think there are a lot of choices. And, it depends on a person's riding. Perhaps also a person's size. Distance?
Most of my riding is on skinny tire 700c wheels. It tows just fine, and can carry as much as I'm comfortable putting on the bike. The distance for my shopping trips add up.
I do have a fatter rear tire on my cargo bike, but generally only use that bike for heavy towing, and thus would have been fine with just about any rear wheel.
I think Yuba may put 24" wheels on the rear of some of their cargo bikes to lower the center of gravity. And, some front loaders use 20" wheels.