Well, remember, I do it a little differently than many scroggers, I like to get the screen as very low as possible and grow out all that understuff whereas most scroggers go a bit more vertical, trim the undebrush and max out just the tops. With my method in mind, when I was in soil, I used 7 gallon deck pots (the plastic square pots for your deck at lowes that you put a potted plant inside so you can take it out and bring inside if needed) and then went even lower when I found some 7 gallon horse feed buckets that were much shallower and wider, like an oil change catch pan kind of, little higher side.
In hydro, currently, I'm very limited with height, we came up with this method by tossing around ideas to squash the plant down and out reducing height and not yield. The resulting yields were just a big benny. Running 3 flowering screens, 4 plants per screen,avg 3-5.5 per plant, we have more than enough meds to keep myself and (Currently) 4 patients supplied.
So in hydro I am currently using 3.5 buckets w/ 1/2" plumbing in a closet with slanted roofs (3rd floor) but if I had room and were looking to really max out my numbers or space, I'd be in 5gals at least, if not 18g rubbermaids and 1" plumbing. I'll go as big as my space allows, if I were outdoor, I'd be in 350g smart pots.
more root-more bud