The general rule is that foot your plant grows, your container should be 2x that amount in gallons. Ex, a 2 foot plant should be in a four gallon container. I know, that's a lot of fucking volume for one plant to take up but that's the general rule so just do your best to follow it. They won't necessarily die unless the plant is ridiculously small, like a 2 foot plant in a solo cup, but they will be stunted from poor root spacing.
I think my 100+ posts giving advice could earn me a rep+ eventually... :sigh: