5 gallon is usually good till flower. The bigger the root system the bigger the plant. Usually its quite hard to move/transplant a plant that has maxed out a 5 gallon container, i plant into 3 gallon, then straight to ground. Anything in containers usually frys in july anyways, this year its gonna be clones straight into ground. If you till up a plot, rather than dig a hole theres no restrictions on how far the roots can grow