Don't grow autos!
Grow photoperiod fems and that way you can control when flowering occurs and only once YOU are satisfied with their height.
Autos really are not for beginners, they need advanced skill and top equipment to get best results.
Photoperiod fems will allow you to learn a lot more about cannabis cultivation and handle small mistakes far better.
Autos are not all what the hype makes them out to be and photo fems are much more stable and rewarding.
Generally, with autos 18/6 is best and lighting quality needs to be top shelf and strong.
Also, topping and defoliation should be avoided when growing autos.