Hmm - confusion, methinks.
According to http://www.reprap.org/bin/view/Main/McWire_Cartesian_Bot_1_2 the x-thread is 19 ", y 11 " and z 9 ".
Except under the bill of materials (BOM) section, where x is 24 ", y is 18 " and z is 12 ". These measurements are the ones I used to mark my threaded rods. Again, I have chosen the longer lengths because I can cut them down as necessary, but once cut too short you're stuck with it!