Why do you want to defoliate??
What do you think it will achieve??
How will it benefit your plant??
Defoliation should only ever be done if your plant grow space is way over crowded and you have extreme issues with controlling high humidity.
Cutting leaves off of plants this size will just stunt them and reduce your potential.
Defoliation is not a required "technique" when growing cannabis.
Just remember.............more leaves = more energy being made = more growth = more buds.
Or to put it another way...........less leaves = less energy being made = less growth = less potential yields.
Also, if you are growing autos, you are much better off using low stress training and not topping them.