Residential sewer pipes can be either 3" or 4" in most states if not all. And I'm about 90% certain that the actual discharge pipe IN the toilet is only 2" (it's 4" in the UK, btw), but I can't find a schematic that includes that dimension on the net. So I'll have to check next time I'm in Home Depot or Lowes.
Guatamalan plumbing may make it necessary to put TP in the trash instead of flushing it....but why create 3rd world conditions on your boat unless you just want to emphathize?