This makes absolutely no sense, what is the point of running larger injectors if nothing else is going to change. Any change in injector size on your ECU is going to require a retune, your not an OBD II where you have some learning leeway. You’ve got a basic ECU that has limited mapping range to work with.
So to answer your question the biggest injector you can run on your OEM ECU without a retune or S-AFR/AFC is your OEM injectors.