I've always used the larger needle and tip sizes based on the area to be covered and the paint being used. When I did T-Shirts I needed both a size that handled the thicker paints as well as flow sufficient amounts to get the desired coverage.
Experimentation on something such as a piece of paper or test model will help you discover what each needle and tip combination will deliver. Practice pactice and more practice so you are fully vested in the capacity of your airbrush!
I too was one of those guys in a big rush to take my new airbrush and use it on a model without first learning what it could do and what I couldn't yet do with it. An otherwise good model, shot because of being in too big of a hurry to spray paint without the knowledge or skillsets to do so.