Gary,
An easy way to consider injector size is the standard GTR has 444cc injectors and produces around 450 BHP max on those injectors.
The same follows through for the 550cc, 600cc, 680cc etc.
Only thing is you should not use injectors above 85% duty cycle and the 450 BHP above is found at near 100% injector duty cycle.
Best practise is therefore MAX BHP should be 85% of injector size.