Approx how long should I cut the z-length tubes? Like twice the height of the machine itself?
According to these assembly instructions, the Z tubes are 7.9" or 20cm longer than your Z axis. http://public.vicious1.de/Assembly_Instructions_v0.2.pdf. Ryan might have more to add.
I would start with 13.5"
I make mine shorter and cut down the threaded rod.