the easiest way to have shorter plants through genetics and lighting. For example, skunk indica will be significantly shorter than the durban poisen sativa. Depending on the lighting you are using, the lights should be a certain distance from the plant. Any higher and the plant will stretch towards the lights. Lastly I recommend you read up on LST (low stress training) and topping/pruning. LST is something I recommend for everyone no matter what they are growing because it helps larger quantity and quality tops. good luck!