Hi,
I have two possible solutions for you.
First, 8.5x13 is a supported size that you can choose from the printer control panel or the Embedded Web Server page. It is called Oficio. The size is not automatically detected, but it can be selected as a supported size.
Second, newer firmware has a fix to allow Custom sizes to be entered over 11 inches. Upgrading to the latest firmware should allow the 8.5x13 to be entered as a Custom size. The newest firmware can be found on hp.com. From the main page, select Support -> Download Drivers, enter M602 into the search box, choose your product, and choose
Cross operating system (BIOS, Firmware, Diagnostics, etc.) from the list at the bottom of the list.
Mike