The septum is usually pierced at a 16 gauge and there is a wide variety of jewelry in that size. Most piercers will use a captive bead ring or a circular barbell ring (which sits in a horseshoe U-shape) to start with. The jewelry can be upgraded to different more fancier options such as a clicker.
Herein, how do you size a septum piercing?
Stretching the septum should only be done with single flared jewelry or a ring in your desired size. You always want to stay consistent with your sizes, jumping between sizes is going to damage the piercing. For example, if you are starting with a 14 gauge piercing the next size you can stretch too is a 12 gauge.
Similarly one may ask, what is the smallest size for a septum piercing?
18g