What MOD rack size to use?

[m_c]

Yes, it's kind of in the name ;-)

And they're not really a good solution for large CNC machines. Any anti-backlash gears I've looked at, the backlash torque is never high enough that the forces on CNC won't be able to over power it.
A quick check of the HTC catalogue I've got, lists the torque for a 50t 2 MOD gear at 2.5Nm. I'm pretty sure a decent cut could manage to overcome that, and on high speed machines, I'm sure the accelerations involved could manage to do the same.

And that's before you consider the additional friction they'd introduce into the system, which depending on the machine, may or may not have any significant effect.

That's not to say they wouldn't help, but you have to weigh up the pros/cons of using them.