How big should the vent pipe be? Can it be too big, We're thinking of using 3inch, would that be OK? We plan to just use this one for the toile, sink and shower. Would it better if we use more than one vent? Its all going to be within feets of one another.

Thanks.

Yeha 3 inch is plenty you can vent a whole house off 1 1/2 " if done properly