The way these things are figured is to look at the charts in the
applicable code book. They've assigned fixture units for various fixtures.
You look at the load, calculate the appropriate developed length, and see
which size pipe to run. By these figures, for example, if there's a full
bath, you can vent a bathtub with 1.5", vent the toilet with 2", and even
wet vent it with a lav. It works just fine. You tie these vents together
and run them to the vent stack (or stack vent). Now if you want to run 3",
it's OK. I guess you could run 4", but it won't work any better.