The size of spray tips has two components; the fan size and the orifice size. Lets use the
LTX515 as an example as it is one of the most common tips for spraying latex paint.
The fourth character, first number (5 in this case) is used for the fan size. If you multiply it by 2 the result is the approximate fan size when the tip is about 12" from the surface being sprayed. So this tip has an approximate 10" fan when the tip is 12" from the surface to be painted.
The last two digits are the orifice size in thousands. So this tip has a .015 (fifteen-thousandths) opening for the paint to go through. Orifice size is directly relational to how many gallons per minute of material will be passed through the tip.
A significant measurement used to determine the ability of an airless sprayer is its "GPM" or "Gallons per Minute". How many gallons per minute a sprayer can deliver determines what the largest tip size is that can be used with it. Also consider that when a tip wears, the orifice gets bigger so in nearly all cases you will select a tip that is one size below what the pumps maximum rating is so as not to go beyond.
If a pump is rated for a maximum tip size of .021 (twenty-one thousandths) you would not want to use a tip lager than a .019