I've read about 1.5 gallons per foot of growth is ideal but I stand to be corrected. I've also read of 12 foot plants growing from 5 gallon buckets. I'd presume genetics also plays a part. General rule of thumb is the deeper/larger the hole the larger the plant, my brother digs 6 foot holes and grows 12-14' plants. He told me a neighbor would dig one giant hole and grow one plant nearly 20 foot tall.