What is the size difference in bullet in the 9mm and 38 special?

There are SEVERAL 9mm cartridges- the most common is probably the 9mm Parabellum (9mm Luger). A 9mm Parabellum BULLET (not cartridge) is typically slightly smaller than a .38 Special bullet- .356 instead of .357. The 9mm is frequently (not always) lighter- and shorter. Other 9mm cartridges (such as the 9mm Makarov) will have different bullets.