The X9 manual lists 2awg cables in the parts list, which works out at 34mm
2.
http://www.superwinch.com/UserFiles/File/90_17371.pdfThe X9 should draw up to 420 amps continuous, which is more than the 35mm
2 cable is rated at, but then these winches were not desined to be run continuously at maximum load. Either 50 or 70mm
2 aught be adequate in terms of basic operation.
70mm
2 cable will have a lower resistance and sap less power than 35 or 50mm
2 cables, resulting better winch performance as well as being able to withstand maximum load for longer without over heating. From a quick google it appears that even 70mm2 cable will not withstand 420 amps continuous. The compromises of increasing the cable size are cost and fitting it in as the bigger the cable size the larger the bends will have to be.