If you start with just-weaned female bunnies and raise them together, 8x8 feet might work for several of them. Does that grow up together will often continue to live together peacefully. Or get an adult doe and one or more of her daughters that have not been separated from her, and let them stay together as the bunnies grow up.
But if you start with adult rabbits that have been living in individual cages, they will most likely fight and injure each other. Rabbits can be quite territorial.
(Of course, individual rabbits might be exceptions either way-- they might fight even if they grew up together, or they might get along even if they grew up separately.)