Generally, it depends on what type of product, and surface you are spraying.
For example: Standard Barrel 515 tips, which is a general-purpose tip. We offer a variety of sizes for different paints and spray patter width needs. The last two digits of the tip size reference the orifice size. I: e. a 515 has a .015-inch orifice. The leading digit (in some cases 2 digits) tells you the pattern width by taking the number and multiplying by 2.
The 515 sized tip starts with the digit 5, meaning that the pattern width is a 10-inch pattern when sprayed 12 inches away from the project surface.
Refer to the instructions on the back of your paint can for determining what orifice size you should spray it with.