There are a few considerations that go into determining the size of fuel injector to use. The first is the engine displacement. This will tell you how much fuel the engine can potentially consume in a given amount of time. The second consideration is the horsepower rating of the engine. This will give you an idea of how much power the engine can produce and how much fuel it will need to do so. Finally, you'll want to consider the type of driving you'll be doing most often. If you're mostly going to be on the highway, you'll need less fuel than if you're constantly stop-and-go city driving.
Once you have all of this information, you can start to look at different injector sizes and figure out which one will best suit your needs. In general, larger engines or those that make more power will require bigger injectors. But there are other factors that can affect this as well, so it's always best to consult with an expert before making a final decision.