Depending on the genetics/strain I grow in either 4 gallon or 7 gallon pots. Roots need room and when they get to the point where they are packed against the edge of the pot and circle around and around it they become less efficient.
Lots of women say that bigger is better and I have the same belief about pots. If you have the room you will always be better off with a little larger pot.
If you are doing sea of green and just going for a bunch of tops that is a different growing style but if you want full thick bushy plants with buds all over you need room for roots.
My family owns a nursery and one thing I have learned about plants in general is that their root mass will pretty much match the size of the above ground plant, minus leaves, so imagine cramming an entire plant into the size pot you are using and then ask yourself if things would be crowded and if more room for roots is needed to have healthy productive plants or not?