Hi Trish, It really does depend on the kind of job you are applying for.  In Development, employers are typically looking for grant writing and letter writing skills.  When I have been in the job market and potential employers ask for writing samples, I usually submit 3 or 4 different samples.  I don't think the length matters all that much.  After all, they don't have to read every word, but give them enough to demonstrate your writing skills and experience.