If you have access to Microsoft Word...look on the drawing toolbar. An oval is one of the Auto Shapes. You can stretch it all directions and make it as large as your paper. You can also set the line thickness so that when you print it, it's easy to cut out.
If you think you'll use the shape a lot, attach it to poster board (or cereal box cardboard) with rubber cement prior to cutting out the shape. Then cut on the line. They last a long time.