If the prose sample asks for say, "15-20 pages" I'd urge the applicant to submit 18 or 19 pages. It will stand out and very positively, as the vast majority of the other applicants will submit the entire 20 pages and use all the little tricks mentioned above. And, if it's a good school, there will be tons of applicants, most of whom will be rejected. One of the reasons (that no one will tell you) is that the person who submits a bit less than the maximum will be viewed much more positively by the reader than the one who crammed every possible space with prose.
Also, a person who fudges with font size, spacing, etc. has violated professional format and that's another big red flag. It instantly identifies the writer as… unprofessional.
Hope that helps! And, good luck!