What sorts of writing samples should be included in my grad school applications?

Should I be using shorter samples, like 5-6 page ones, or my longer 10+ page term papers that I've written? Do they need to be directly related to the field I am applying for? For example, I would like to apply for Medical Anthropology, and I took an undergraduate medical anthropology class, but I think my papers in my hunter & gatherer class was better. Would it be a good idea to send in a paper that relates, or a better written paper? Thanks!