I run into this when I'm teaching first year writing, too. Students will ask things like "how long should the introduction be", and the answer isn't "one paragraph for a five page paper and two for a ten page paper". No, the answer is "long enough to introduce the topic and give any background information your reader needs, not so long that you start to get into your body arguments before you actually get to the body". They ask "so, is three pages too long" and I can't answer in any meaningful way without actually reading the paper. It might be. It might not. Pages are the wrong yardstick with which to measure, because it's the content that matters.
For similar reasons, the infamous advice to cut adverbs is useless when accepted and deployed uncritically. The more useful rephrasing might be: Persistently using adverbs as a shortcut, in place of giving some meaningful description of the characters' actions or thoughts, will make the story shallower, and adding them where there are already sufficient cues will make the story sloppier. The more useful initial phrasing might have been: Identify the techniques you are prone to overuse and remember to pay attention to those while you're editing.
Of course, that doesn't sound nearly so satisfyingly solid and simple, does it? It's not as catchy as "The road to hell is paved with adverbs". It sounds less like "fewer than three pages" and more like "not so long you start writing body arguments".
Prescriptive advice isn't always wrong, but it isn't going to be right, either--again, those are the wrong yardsticks. It universalizes the particular way of writing that worked for one author/reader or even a group of authors/readers. It borrows the false authority of absolutism instead of putting in the work of self-examination that might yield the far more useful explanation of why, in that particular case, a particular writing approach worked.
So to anyone who is tempted to write a how-to or a this-is-better: try to remember that your view is specific and particular, not universal, and do the 'why' work. It's just as necessary in non-fiction as it is in fiction.