I mean it does also depend on the type of plants you wish to have whether indica, or sativa, or a hybrid. Remember that sativas tend to grow much taller, and indicas literally a bush. Just veg them until you are comfortable with the veg height then put them into flower, with LST during the veg phase if you wish. You can also top each different budsite during LST to just get bunch of colas and a bushy as hell plant.
I would say if you have around 5-6 feet of space for them to grow about 3 feet tall.