I assign minimum page requirements to stick it to my "do-as-little-work-as-possible" students. They get back at me by making me read what they write.
No, really, though. My students always want to know how little they can get away with doing. (I say "my" as if it's unique to me. Everybody's students want that.) In most of my formal writing assignments, I tell them a minimum of 3 pages. I specify margins, acceptable font sizes, spacing requirements, everything. That's what my high school English teachers mostly did; that's what I do. I tell them that when they get to be crack journalism students, they will be able to write comprehensively about a topic in fewer pages, but for now, just do it.
Students go to extraordinary lengths to hit the page requirement, without actually doing any more writing. It's amazing. Instead of a punchy conclusion, the last line of the paper is occasionally a drawn-out, run-on monstrosity with more clauses than a sack full of catses. (I've been waiting years to write that joke.) I imagine that that's why the model shifted to the introduction--3-body-paragraph--conclusion model; that way nobody has to care what the page requirement is. (Weirdly, though, everyone still does. Evidently, this includes the people who assess the ACT writing section.)
Once I get an efficient system for turning papers in digitally (Google Docs, Moodle, I'm looking in your direction), I can use a word count system as a way of judging minimum lengths. But the point is that I hate that kind of nonsense. It's stupid game-of-school stuff. There's nothing magic about three pieces of paper with ink on them, especially if the third one has exactly seven words printed on it; there's nothing magic about 1500 words, especially if 400 of them are "very". It's just that most students stop thinking far too soon without some kind of minimum requirement. The minimum requirement doesn't seem to be any help, either.
Maybe in the future, when I get better at teaching and grading by rubrics, I'll be able to show students that a minimally good paper will probably take up 3 or more pages, but that I'm not grading by weight. In the meantime, I have 35 pounds of papers to grade.
Readers: How do you ensure that your students put an adequate amount of thinking into your writing assignments? I'm talking Tier-I, fully functional, basically good high school writers. Everybody knows that some students will never do that, and some will only do it when people sit next to them and say, "Get back to work" every 15 seconds. I don't mean them.