with good quality fuel injectors, you can go almost as big as you want.What requires the upgrade? Forced induction? I think duty cycle will decide the size you need so you need some numbers to get something close to what is right.
I don't have larger injectors in my truck, it's stock. I have gone larger in cars and other vehicle's I have owned because of engine mods
I think holly might have a section in their tech area that talks about how to figure injector size