How to find out the size of a tongue ring bar?

The barbell used is scaled to the tongue being pierced to ensure there is room for swelling and discharge. The professional body piercer will ensure that the right size bar is used and every tongue is different, there is no specific size (only a ball park guess nothing more). To find out how long your current jewellery is would involve you removing the barbell and measuring it with a ruler to know what size is in the piercing right now. If you do this be sure to use care in handling the jewellery and ensure the piercing is not contaminated by inserting dirty jewellery into the piercing.