Like most teachers, I think, I don't like when students ask how long a paper should be, especially when it feels like what they want to do is the minimum. You probably know the sort of paper I'm thinking of. The student gets a page minimum (n), writes blather for n-1 pages, and one part of a sentence onto n, and then stops, pretty much mid-whatever, because they've done the page minimum.
It's not most students, but it happens, right? And that's always my worry.
What I want, in my ideal world, is for students to start working on what they want to say, and to say it well until it's said, and be done.
When the shoe's on the other foot though, when I'm writing a statement for a grant or something, I totally want a page limit.
If you tell me, write two pages explaining your project for a grant, I'm pretty happy. If you tell me to write no more than three pages explaining why my colleague should be tenured and promoted, I'm pretty happy. But if you shrug and say, just write as much as you need to? I'm unhappy.
If I know that everyone is just writing two pages for that grant, then I know I'm not going to turn in something wildly out of proportion to what others are doing, and mine won't look either overwhelming and too big, or like I didn't care enough to really explain.
The same with the tenure letter; if I know no one's going to turn in a 12 page letter to get their colleague tenured, then I won't feel like my three pages are unconvincing because I could only write three pages.
(Of course, in either case, I'm going to work like the dickens to say a lot, as clearly and convincingly as possible in the allotted pages.)
How about you? Page limits for student writing (even as suggestions), or no?