No point using a 20mm diameter screw on Z as it's so short that the critical speed of a 1605 screw is plenty. Use 1605 for Z and you'll get better acceleration.

For the other axes I agree with the choice of 20mm screws if you want to get the highest speed from your servo motors. However if you want the best acceleration then using 16mm screws would be better, but this will of course lower the top speed.