Size 1 is the smallest size soccer ball. Typically, they have a circumference of 18 to 20 inches (or a diameter of 5.73 to 6.36 inches). They're typically inflated between 8.5 and 15.6 PSI at sea level. A Size 1 soccer ball weighs about 7 oz, and is a great ball for young athletes under the age of 4. You may see these called "mini soccer balls", and they can also be used to work on footwork, ball control, or even as collectible souvenirs.