Fuel injector size determines how much fuel is able to be injected into the intake system by the injector. Normally, fuel injectors are sized by cubic centimeters per minute of fuel able to flow through them. Another sizing protocol is pounds of fuel per hour per horsepower produced. There is a set of calculations and formulas that can be used to determine the proper sized injectors for your car and the amount of horsepower that you want your engine to produce. One of the easiest sets of formulas to use can be found here. Another good calculator can be found here.
For more info, see Things to Know When Considering High Performance Fuel Injectors