Peggy Hall (aka "the head mistress" @ http://www.sailboatowners.com/forums/menu.tpl?fno=31 ) provided invaluable info when I was replacing the
holding tank system and hoses on my Ericson 31.
In short, figure on 2 to 3 gallons per person per day, The system needs lots of ventilation to allow areobic bacteria (non-odiferous) to to outnumber the anerobic bacteria (smelly kind). Peggy was also very helpful in providing troubleshotting information as I was debugging my new system and winterizing it for the first time.
Good luck