The only time I give students a minimum or a maximum page count is when I ask them to write an introduction to their papers. I usually restrict introductions to a one or two page maximum, because in philosophy introductions should be “all business,” concise, and to the point.
But beyond that I impose no minimum and maximum page counts. Do I have a sense of how long a good paper should be on given topic? In most cases, sure I do—but I won’t share that with students. The reason for this is that page counts are arbitrary. Look, we all write differently. Some students take longer to write a good paper, while others can do so in fewer pages. The proper answer to how long a paper should be is however long it takes you to answer all of the questions posed by a topic in adequate detail. All other things being equal, if a student has done this they will earn an “A”—regardless of the page count. The focus should be on your ideas and your argument. It should not be on the number of pages you are writing.
The imposition of minimum page counts are especially pernicious. Whether imposed by the instructor or by the student herself (“a good paper can’t possibly be less than six pages so I must write six pages”), this often results in badly overwritten papers because the student starts repeating themselves to make this arbitrary page limit. Ideally, each sentence, each paragraph, should contain a new meaning or an additional idea. But if you’re writing to reach some imagined page minimum, you’re likely just repeating the same ideas over and over again to no argumentative purpose.
What all this means is that longer papers are not necessarily good papers; but neither are short papers. Write to answer all of the questions posed by a topic in adequate detail, and let the page count fall where it may.