Depends on several factors friend. They usually will grow 2-3x as large after flowering period commences. Depends also on your objective. My friend grows many plants that are 12 inches at harvest. Fat, fat, fat colas. I veg mine at 12" and end up at 32" at harvest. I am going to start going earlier though, because I like the compact results between his and mine. I will probably shoot for a happy medium though of 18" next round, so I will change light cycle at 6". If you are growing only a couple though, I would probably let them get bigger. Not to big though, light is exponentialy less efficient the further away it gets. That is why the buds at the bottom of a plant are less dense and small.