I don't subscribe to that 1'/gal theory, unless you are growing in a soil with organic material and nutrients built in.

IMHO if you are using liquid nutes, in a medium devoid of nutrients (like pro-mix), the size of the pot does not really matter. As long as the plant gets what it needs to grow, it will grow as big as you let it. I've grown and flowered 4'-5' plants in 4" square pots and if you looked at them from stem up, you could not tell they where in itty-bitty pots.

Now I don't really reccomend that big in such a small pot because it causes other problems. Like needing a watering at least once a day or more and when they get that big they want to fall over 'cause their feet are so small, get very top heavy you know.

(I can hear nashing of teeth, and an angry mob headed to thier keyboards)