The size of the tip will correspond to the amount of solids you are spraying.
For example, epoxy primer requires larger than basecoat, and basecoat larger than clearcoat. Clearcoats are around a 1.5 usually, and basecoats are normally 1.6-1.7. Check the technical bulletin for the paint you will be using. They also give air pressure at the cap, time to dry, recoat, tape etc.
joe.