I started with 1 gallon then on to 3 and finally im in 9 gallon. 5 weeks veg, they are all standing at around 15 inch now and they will probably end up being 5 foot tall at harvest. But this depends on the strains you are growing. I would say unless your growing a plant that you want to end up like a tree then anything up to 15 gallons should be good for a plant 6 foot tall plant. Anything more and you will have to get your chain saw out to crop them...lol Again there are things you can do to stem the growth of a plant. i.e super cropping LSTing etc etc. I herd somewhere on here that the main tap root of some plants can search for water up to 6 foot under and if you go on the principal that if the main root can go this deep then the roots coming off this will be almost that wide too.
hope this helps.