I could live nicely on a 30 footer, and if alone, could be comfortable with a 22 footer. (But I've also lived out of a backpack for a months at a time.)
What you can live on depends entirely on what you consider indispensable equipment. My wife considers a television as indispensable, I may watch it two hours a month, (my record so far has been four hours one week on a mythbusters marathon) so for her I need to have room for a TV/dvd etc, and the required equipment to run it.
Make a list of what you NEED, determine what is needed to use what you need, and then figure out how much space is required for that and the required support equipment. (if you need a refrigerator, you need a power source large enough to run it, if you're happy with an icebox, you need one large enough to hold the food you need along with the ice required)
Another determining factor is how often you can re-supply, if you can restock every couple of days, you can get by with less storage space, if you can't re-supply for a week or two at a time, you need more room simply to store things.
So minimum size depends entirely on what you need.