Best way to answer that is to take your grow space size, figure out how high you will be able to hang your lights at flower at max height. this should give you the top height you'll want your plant to be. a plant can grow 2-3 times in flower so take this into account and start your flower when they are about 1/3 the size that your looking for just to be safe! Don't want them growing into your lights. Hope this helps! Happy growing!