15 gallons has worked for me....
...but bigger would be better. It has been fine for me alone for a long week of sailing, and has just about worked for three (teen-aged daughter and wife with me) for a long weekend. If we had room for much bigger, I think 20 or 25 gallons would be great, but on our 27' boat this is all I could fit without considering a custom tank in the forepeak. Interesting question. I'll be curious to see how others answer.