Even smaller- just enough so that there's about an inch of fresh, light soil all around the root ball. So from a 3" pot, you'd go up to a 5 or 6" pot, whcih is a quart, or equivalent to a 6" tall section from a HALF gallon milk jug with the top cut off and of course punched all full of holes and covered in duct tape. Or you could just look in the cellar for used perennials pots, but I'd rinse them and run through the dishwasher first in case of pests.