A typical "ballpark" calculation for injector size is:
(Flow * max duty cycle * cyl #) / BSFC for the motor.
In other words, multiply the flow rating of the injector (lb/hr) times the maximum duty cycle (typically no more than 80% for reliable operation) times the number of cylinders. Then divide this value by the BSFC (brake specific fuel consumption - Google it if you want, it's a wierd value). BSFC for a normally aspirated motor is about .5. Supercharged - .6, turbo'd - .65.
So, for my truck for example, the stock injectors flowed 19lb/hr. I used 80% duty cycle, and it's a V8. This was also in prep for the turbo, so I used .65 BSFC. (19 * .80 * 8) / .65 = 187.5 - the stock injectors were good for roughly 190hp under boost. Obviously, I knew they had to go. An N/A 5.0 makes more than 190. Went up to 39lb/hr's - moved the ceiling up to about 385hp.
Keep in mind these values are by no means exact, but it will let you know if you're in the ballpark. Or way the hell out for that matter! Are you turbo'ing a 2.3 or what?