Tankless water heaters are much smaller than traditional models, so they can be installed almost anywhere, including outside the home. A traditional model has a large tank that generally stores between 30-80 gallons of water. They are often 3-6 feet tall and take up 2-3 square feet of floor space. A tankless water heater is often wall-mount, and is about three feet tall, two foot wide, and a little more than a foot deep. Size does vary be brand and model.