A sufficiently strong machine

[Jonathan]

I want quite a heave duty machine for milling stone/ally

Question. Would there be a benefit from using larger motors Nema 34s or 42s or might it be a problem

For the machine in this thread, there would be no benefit - in fact it would probably make the machine worse. Larger stepper motors tend to not spin as fast, but output more torque. If the smaller motor can output sufficient torque to obtain the required aceleration and speed, there is therefore no point using a larger motor as it will just cost more and sometimes not go as fast.
You would have to be building a huge machine for Nema 42 motors to be the correct choice. See this thread for a more detailed explanation.