Sample Prospective Cover Letter When A Prospective Employer Asks For A "brief, Unedited Writing Sample," How Long Should It Be?

When a prospective employer asks for a "brief, unedited writing sample," how long should it be? - sample prospective cover letter

I am applying for a job in the nonprofit sector and the organization calls a cover letter, resume and a short written test unreleased. I graduated with a Master's in May 2006. Should I an excerpt from one of my papers? Or should I write a brief opinion on a topic that interests me? Finally, how long they should be represented? Thank you!