Assuming soil, I would use 5 gal because they should be at least 4 ft tall when they are done if flowered at 24"(depending on strain, of course), and thats gonna need a lot of roots. I flower at 14-16 in. and fill a 3gal with roots...in coco.
Pretty open end question. but once my seed plants are established, I go to a 1 gal. pot then a 3 gal. pot then flower in a 5 gal. pot. I use plastic grow bags when using dirt. I don't have much luck flowering in dirt though. I like a full root system, but that my be why my plants always grow to tall. LOL.