if you have a plant that stretches alot like a sativa you shouldnt get it bigger than 2 feet , if you have a indica that has minimal stretching 3 feet would be good that way they wont get to out of control youll have 3-4 foot plants . but its totally up 2 you , like what strain your running what light are you using , how patient are you ,whats your time frame , are you going to top or lst? , etc..