I was just thinking about this again and, while its clear many people would find a 20' boat to be hell, I really think that ones own experience should be your (ops) guide in deciding.
Im am not a large person and usually live very simply. I would probably get along with a 20 footer just fine. Clearly a single
person wont always be on the boat and for most of us our home time doesnt require a great deal of space.
A 20 foot boat doesnt need a great deal to heat it within comfort tollerance. Depending on where the boat is kept, just making morning coffee might be enough to chase the chill.
And a single
person probably will keep cooking
Yes, this is a bit of boat camping but, depending on ones nature, it could be quite doable for quite some time.
For me, it would be just fine. I grew up in a house in the Colorado rockies without central heat and spent my youth camping and backpacking. A 20 footer, especially if I was in my 20s would work quite well.
But you do really have to consider who you are and go from there.