Plumbing Vent Size

Here is an interesting tidbit. You dont need 11/2, 2, or 3 inch vents unless local code requires those sizes. Most codes are pretty ambivilent, stating that the vent should be sized for good function. Thats way open to interpretation. Many modern houses only have 2 inch vents, why waste the time and resources making three sizes of vent when you dont need them.
For most applications, all you really need is basically a 1 inch pipe. That will allow far more air than needed to displace water when its flushed out the home. But the larger vents allow easy plumbing snaking if there is a plug, so thats why they are sized that way.