How to choose a correct size pot

Ok so I have a 4x4x6.5 grow space. I was wondering how big of pots I would need to fill the entire grow space with lets just say 4 plants. What size pots should I use? I currently have a plant it's a month and 2 weeks since I started germination. The plant roots are already growing out of a 11x11 pot. How big of pots should I get? 10gal?

i like my gallon pot holds alot and more room for roots to grow too mate

2 gallon pots should suffice.

Dude 1 gallon is way to small for this hedge bush... It's roots are growing out of a 3 gal pot already...

Should I just let the roots grow out and let them die or should I get bigger pots?

Bigger pots, maybe 5gal.

The plants measure 17" high and is 20"x24" It actually only 1 month and 3 days since I started germ.. Is 5 gal going to be big enough though by the time it's done flowering?

1.5 gallon is plenty. people grow 5 foot bushes in dixie cups...