What would the highest recommended CC injecters be to put in my 1.8 08 honda civic ex be and how much would it change the gas mileage?

 

Of course changing to larger injectors would change your fuel economy numbers!!

You also have to get a cold air intake or go forced induction to restore your stochiometric ratio (14.5:1 Air/fuel ratio ) which is the ratio needed to properly burn fuel.

I have no positive answer for the injector capacity, but 450cc to 750cc would be the range you need to look at.
S2000 or Si injectors if you want to keep them in the family, or maybe Pilot. Ridgeline or at the outside NSX injectors.