A recent post in Tomorrow’s Professor treats the forces converging on that often irritating but essentially benign student question: “How many pages?” Here’s the lead:
He said, “How many pages does that paper have to be?”
She said, “As many as it takes to make your case.”
This exchange is pretty common, and annoying. The student is trying to set the boundaries of the assignment and is probably annoyed with the vague response he got from the instructor. The instructor wants the student to learn how to make a good argument, and is probably annoyed that the student seems to be focusing on quantity rather than quality. But there’s a motivational theory that might help each party understand the other.
Teachers, read the full post for insight into students’ impulses for this question, and your own motives for deflecting this question, if you do indeed deflect it.
A note about the image that illustrates this post: That’s a photo of an 8,600-word draft of an essay I had to cut to 5,000 words in order to submit it to a journal. I’d simply like to convey that page lengths exist everywhere in the professional world, and making students guess at our expectations for the length of a paper or presentation is inconsistent with the submission guidelines they will someday encounter in their vocational and even avocational lives.
leaf stitch word 
I agree that teachers should give more guidance. However, just as there’s a big jump between the teacher’s and student’s perspectives in this scenario, there’s also a huge difference between learning to write and writing for publication.
So I lean more toward leaving things open-ended for students, allowing them to feel some discomfort. Even though they’ll have word limits and deadlines to guide them in the “real” world, there’s a lot they’ll need to figure out on their own. Writing assignments should help them learn how to make some of those decisions, or at least give them opportunities to deal with that discomfort so it’s not a surprise when they have “real” deadlines.
To that end, the teacher in this scenario should help the student come up with a minimum and maximum word length, since s/he surely must have some idea of how many would not be enough and how many would be pure torture to have to read.