since you are growing outside, the bigger the pot, the bigger the plant will be. i use mostly 15 gallon pots, tho i have bought this year some bigger ones from home base, they ran bout $20, and i am estimating they are like 25 or 30 gallons. Then i went to the hydro shop near by and found some huge hydro pots, just plain black plastic pots. Them i put two plants in each as i didnt want to buy so much more soil.
You will be needing to get a bigger pot tho, soon.
i go four inch, to one gallon, then the 3gallon or 15 gallon, depending what pots are avaliable.
good luck
plants look real nice