There are a variety of balloon sizes, but the most common are 9-inch, 11-inch, and 12-inch balloons. There are also speciality sizes, such as 6-inch and 18-inch balloons. The size of the balloon you need depends on the event you are decorating for. For example, 11-inch and 12-inch balloons are typically used for balloon arches, while 9-inch balloons are better suited for balloon bouquets.