The writing sample should represent your current abilities as a writer. It should be no more than 4,000 words long and should be in standard manuscript format.
If you have a complete short story that fulfills these qualifications, then that is often the best thing to send. If you write very short pieces, you may send two, if the total is under 4,000 words, but no more than two.
If you only write longer stories or novels, then often the best option is to send the opening section of the work with a brief synopsis of the rest (the total of excerpt plus synopsis should be no more than 4,000 words).