Adding shower

When adding a basement shower to an existing half bath, what is the minimum required vent diameter if the vent will be designated only for the shower?

I don't see why you can't keep all this going in one thread like everyone else does...
Where do you live and are they under IPC, UPC and are local ordinances over riding anything those two code institutions state, City hall can help you if you don't now those answers.

New York Hudson Valley 1 hour north of Manhattan