Lenght of sensor cable for Mad Rat/rear motor?

[Giggs]

I'm upgrading the motor that came with my rat standard kit (which does not have a sensor cable). My new electrics (nosram matrix/evolution) comes with a 5cm sensor which is too short for my rear motor setting.

Before venturing out there an buy the wrong lenght of sensor cable, what would be a good lenght for a sensor cable for a Mad rat/rear motor set up (the speedo is place towards the back of chassi)?

Also, is there any real difference between the different types or the cheapest will do fine (just bashing not racing)?