I grow three large clumps of cilantro in a rectangular box that is approximately 10 inches by 10 inches by 30 inches. The plants grow fine and produce lots of cilantro. Last year I let my pasley and my cilantro go to seed. Am getting lots of volunteer plants from both this year. I have found that early planted cilantro goes to seed very soon, my earliest starts are already flowering, so IMO succession planting is the only way to go, to get a continuous supply of cilantro.
This year I have six clumps of cilantro in two rectangular boxes, and then have new seedlings of various sizes growing in about a dozen one gallon nursery pots. As the cilantro in the rectangular pots goes to seed, the plants will be replaced with the fresh starts. I plan to do about three different age groupings of cilantro, to ensure a good supply until frost. Last year we started saving our own seeds, and they have seemed much more viable than bought seeds.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.