What is the stock Pontiac grand am exhaust piping size?

The stock exhaust pipe size on a Pontiac Grand Am varies depending on the year and engine. There's no single answer. You'll need to specify the year and engine displacement (e.g., 2.4L, 3.1L, 3.4L, etc.) to get an accurate answer. Even then, there might be slight variations depending on the specific trim level.