The two most common size drip tip sizes are called 510 and 810. A simple rule of thumb: 510 tips are narrow and 810s are wide. The original standard for many years was the 510. Today they are used mostly for mouth-to-lung and restricted lung vaping. Some 510 drip tips may have a wider bore than others, but all are around 8.5 mm in outer diameter at the base.
The 810 drip tip was originally popularized by RDAs like the Goon and Kennedy. Becoming the standard for cloud making devices, 810 drip tips typically have a significantly wider bore than 510 drip tips and are around 12.5 mm in outer diameter at the base. The 810 drip tips are now common in most popular sub ohm tanks and drippers.