Plumbers?

3" is minimum. Normally you can cut out 4" of the top and bottom wall plate to run a 3" line. Shower drains are 2" and sinks are 1-1/2" which on a bath sink are reduced to 1-1/4" at the sink hookup. A vent pipe is required within 6 horizonal feet of a drain to prevent suction from sucking the water out of a trap and allowing sewer gas into the house.
While staying at a B+B in Stratford on Avon (England) I saw a toilet which the owners had installed to expand their bedroom capacity that had a mulcher or something like a disposal behind the toilet that ground up anything that was flushed and the output line to the sewer was a standard 3/4" copper pipe. Very ingenious way to add a bath anywhere in a house without having to run a 3" line. Check it out

http://www.saniflo.com/SANIGRIND.aspx



Hope this helps, Jim