The print area is 8.5x3 for the 11oz. Mug and 9x4 for the 15oz. Mug. It is then centered and wrapped around the mug.
As you can see, the print area (GREEN) will be printed on the Mug. Anything outside that area (RED) will not be printed. Remember, the print area for the 11oz. Mug is 8.5x3 while the print area for the 15oz. Mug
Above is an example of offset position for Mugs. As you can see, you can achive a left, center, or right print by positioning the logo or artwork accordingly on the 8.5x3 document size.