A full size mattress, also known as a double mattress, measures 54 inches wide by 75 inches long. It provides enough space for one or two people to sleep comfortably, making it a popular choice for guest rooms and smaller bedrooms. This mattress size is larger than a twin mattress but smaller than a queen mattress, offering a balance between space and comfort. Many people choose a full size mattress for its versatility, as it can fit in various room layouts while still providing ample sleeping area.