Or...
Minimum Injector Size (READ NOTE ON BOTTOM) = [max airflow you want/expect (lbs/min)] / [AFR you want to run] x [454 g/lb] / [.76g/mL for pump gas] / [# of injector] / [max IDC you want to see]
Or simplified:
[max airflow you want/expect (lbs/min)] / [AFR you want to run] / [max IDC you want to see] x 149.34 (for 4 injectors)
e.g. You want to flow 30 lbs/min and run a 10.5:1 AFR and want to keep your IDC below 80%.
30 / 10.5 / 0.8 x 149.34 = 533 cc/min. Round up and you get 550 cc/min injectors.
Taking note that 1 lb/min of airflow is roughly equivalent to 9-11 crank horsepower (so say 10), you can use this to estimate injector size based on your power goals.
NOTE: The nature of these calculations are highly theoretical and thus you should apply a safety margin at all times when using these calculations. These calculations do not take into account every real world situation and there are about a billion things that could screw these numbers up. On the other hand, you get a good approximation of what you need. Always use a safety margin of 10-20% if you actually do use this (meaning divide your final answer by .90 or .80, meaning the above 533 cc/min would go to 592 cc/min or 666 (zomg!)cc/min depending on what you divided by).
This also assumes that you run 43 psi base fuel pressure which is what almost every injector company gives out their ratings based on. E.g. RC 550cc injectors are meant to flow 550cc at 43 psi base. If your base fuel pressure is 33 then they will perform like 481cc injectors.