Plumbers?

I've seen two inch turds*, so two inch is out. Go to your local home depot or lowes and ask their expert, so a) it will work and b) you'll be in code just in case there is ever an inspection. I think waste out is 4 inches. You must have a vent or it will not drain properly.




* I am a teacher, and have had to go in and flush when the kids come back out of the restroom running from the "turd monster" seems to be some sort of contest to see who can leave the biggest turd, IDK!