I have two discharges, one for sink and another for shower. I use one 4 gallon container for shower and one 4 gallon for sink. I empty them every day before they fill up. With the two together I use about 6 gallons of water a day(with navy type showers). If you have a single discharge for both shower and sink then you would probably want at least a six gallon container for grey water. Might want to get one with wheels on it as 6 gallons of water is about 50 lbs. or just empty before they fill up.
2012 Rockwood a124