Just read an article in PM, advocating nothing larger than 3" for all vertical and horizontal mains, including the run out to the tank or sewer. The authors reasoning is that with low flush toilets and the new HET's there is so little water mixed with the poo that for separation not to occur, smaller pipe sizes should be used. Not sure how I feel about this, but I'd like to see some data. (none was supplied unfortunatly)