does it harm a plant if it grows to the top of your growbox?

ive seen countless people with homemade growboxs that are only about 18 inch high. the plant would grow to the top of the box and spread all over the place, is this ok for the plant if you just position the lights away from it so you dont burn the leafs?

Try some Low Stress Training (LST)

This is just tying the plants down. Look it up, it's easy and you will get a better yield.

You will also make better use of your space.