Respond to the questions or prompts for each school. These can very widely from school to school.
Read the web pages for each program carefully. Sometimes further information about the essay or even a different set of questions is provided there, rather than with the graduate application materials for the larger university.
Take care not to write your essay as if it were a retrospective, i.e., "I have wanted to be a marine biologist since I was seven. Then in middle school I went on a whale watch and my interest was confirmed. Then I did this. Then I studied that . . . etc." This is too much about your developmental process and presents you initially as a seven-year-old. If you choose to tell how your interest developed, you need to write the account with maturity and support it with accomplishments.
Follow the directions and answer the questions. If you are considering going over stated limits on number of words or pages, call the school and ask how that would be seen; some schools are fine with essays that go over to a certain degree, but some will view that negatively. In terms of length, if no direction is given, the default is generally two pages, double-spaced with clear paragraph spacing; some schools are fine with two pages, single-spaced. If limits are not stated, or if you plan to go over, the prudent thing to do is call the program and ask.
You want to give the impression that you will fit in with the department/program to which you are applying. Hampshire students tend to be interdisciplinary in their approach to any study. If you are applying to a geography program, however, it needs to be clear why you are choosing geography and not urban studies; if applying to law school, why law and not social work; and so on.