That's not a problem. Start flowering between 6 to 12 inches and they will double in size, any bigger and top light will not get through the canopy. This is the same for any inside grow.That's where some flouro side lights come in if needed. Most hydro growers learn this the first grow or so. You can also trim side branches if they get too tall or some crop the top buds when ready and let the lower ones go a week or 2 extra for the lower buds. Hydro is total control. The oxy in the water only helps the whole plant, I have not seen alot of side branching unless I trim the side leaves to promote side branching.