When I still grew outside on my deck I used 15-gallon pots, and larger in some cases. I am in an area with a fairly long growing season and if I got my plants in early with as long as they would veg before flowering, by the time I harvested and removed the old rootball and soil there would be roots using the deepest parts of the pots. The roots were not packed in but then plants do not grow well when root-bound and when root-bound they are subject to any number of various problems. One of them is stunted growth and it was said that the plants the thread is about have had slowed growth.
When picking a pot size keep in mind that the above soil growth of plants is always pretty close to being in balance with the below soil growth. The shape of growth is of course as different as it could be in appearance but in overall mass plants stive for a 50/50 balance of growth above and below the surface. If they lack space below the surface the portion above the surface will never grow to be all it otherwise could be. It still might be enough to be impressive to the person growing the plants, but it will still never be all that they otherwise would have been.
Something else to keep in mind is that while you can very easily pick a pot size that is too small it is impossible to pick a pot size that is too large as long as there is the needed space, so never be afraid of going bigger.
I always tell people that when you pick pots THINK LIKE A FEMALE and always remember that BIGGER IS BETTER. If you do that, just like a female you will find yourself saying OH MY GOD, OH MY GOD, OH CHRIST, OH GOD, OH GOD .... but your excitement will be over your plants and yields rather than because of a massive .... well, let's just say that you will be very excited and very joyous over your experience.
