The print area is 8.5×3 for the 11oz. Mug and 9×4 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 8.5×3 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 8.5×3 document size.