While sofas, including modern sofas, come in many different sizes, the most common length is about 84-86 inches long. In addition, the average sofa has three seats and is about 40 inches deep and 33 inches tall. With that in mind, the ideal sofa for your space may be bigger or smaller than the average. For example, if you need a four-seater couch, the average size is around 90 inches long.