Typically there are three different mm (millimeter) sizes when it comes to your pieces. The smallest, 10 mm, the most standard, 14mm, and a bit larger, 18mm. There are also two types: A female and a male. The female fits around the male.
This diagram helps show the looks of the three sizes of bangers in both male and female.
Something like a titanium e-nail is very perfect for fitting any size. The edges that are shown go into your piece, and will stop at whichever one fits the mm of your bong. The very bottom starts at 10mm, then it works its way up to 18mm.
Displayed here is a penny used to compare sizes between the different mm of bowl pieces, both male and female.