There are 2 ways (actually loads of them includin the swa scale...)of measuring the size of a cable and manufacturers seem to swap between them at will... 1st is cable size and the other is conductor size. If yours is measured as cable size then 16mm2 includes the insulation, so nowhere bear large enough for a winch. However if it's s 16mm2 conductor then that is probably big enough... ( not having google it to check). However the longer the cable is the larger it wants to be. So although 16mm2 conductor is ok for the winch to the solenoid, out of caution I'd go at least double that for the feed and the earth.