I used a 6-quart slow cooker. If you aren't using a large piece of meat, a 4-quart will do just fine. In a mixing bowl, combine all of the ingredients and whisk together so the cranberry sauce is broken up and the Dijon is evenly distributed. Pour over the top of the meat.
Cover, and cook on low for 7 hours or on high for 4 hours.
I cooked 3 pounds of frozen oxtails on high for 4 hours in a 6-quart, and then they stayed on warm for another 2 hours before serving.