My general rule is that a function should help keep things DRY and/or easier to test. Functions should be _reused_.
If your code does a 30 line operation exactly once, it can be inlined. If you want to move it into a function to validate error handling, that is great and now your unit tests help validate code paths.
The fact that is is 30 or 100 lines is immaterial. Making "micro functions" to keep each small just makes it harder to follow the stack. Readability is key and micro functions hurt readability.
DRY is overrated, and lesser engineers can make a hash of your entire codebase in the name of DRY (and don’t get me started on DRY tests. Fuck me.)
In fact the Rule of Three all but says that it’s okay to repeat yourself once, twice is not good, and three is right out. That’s substantially more repetition than DRY prescribes.
Instead, and I admit this sounds a little vague, the code should say what it does. The bigger the thing it’s trying to do, the harder it is to state that clearly, or to verify, so you break it down into separate concepts and string them together. And then you notice how awkward it is to state the same thing many times so you are highly motivated to reuse those statements, refine them, and make them as accurate as possible.
Inlining frequently fails this test because you lose the boundaries of the “thing” that is happening, and people start slipping non sequiturs in, start rambling, which makes it very hard to follow their reasoning.
If you can’t follow their reasoning, you can’t defend it. You won’t defend it. And pretty soon it doesn’t say what it used to and that’s where you get regressions. So if you care about that, you want to write your code so that they understand, in which case breaking your intentions becomes pretty indefensible.
If your code does a 30 line operation exactly once, it can be inlined. If you want to move it into a function to validate error handling, that is great and now your unit tests help validate code paths.
The fact that is is 30 or 100 lines is immaterial. Making "micro functions" to keep each small just makes it harder to follow the stack. Readability is key and micro functions hurt readability.