i have concluded a unit needs to be large. Not a single class or function but a large collection of them. When 'archtecture astronaughts' draw their boxes they ar drawing units. Often thousands of functions belong to a unit. even then though often it is easier to use the real other unit than a test double.
You can make your own definition of unit if you like, but the usual definition is the smallest whole piece of something - a unit of currency, a unit of measure, etc.
If your unit is thousands of functions wide then you have a monolith, and there are widely discussed reasons why we try to avoid those.
i have concluded a unit needs to be large. Not a single class or function but a large collection of them. When 'archtecture astronaughts' draw their boxes they ar drawing units. Often thousands of functions belong to a unit. even then though often it is easier to use the real other unit than a test double.