A square fire pit should be between 36 inches and 44 inches in diameter in most cases, but it’s really up to you and your vision for a truly unique patio, backyard, or deck.
You have to consider your goals. If you want something that’s going to keep things cozy and close for just a few guests, then you’ll want something on the smaller side, but if you want to have large gatherings, go with the bigger fire pits.