I'm converting a 1984 Proline Walkabout Cuddy (23' long) into the world's first electromagnetic powered houseboat. Aquatica will be around 25 feet in overall length including the bow deck.
The living space is 19 feet by 8.5 feet and encompasses approximately 152 square feet of space. I'm 6'2" so the salon ceiling is a vaulted 7 feet tall with a skylight built into the center. The entire forward compartment has this height and configuration.
The aft compartment which includes the head, galley, and two equipment spaces for air conditioning and dehumidifier on one side, and washer/drier on the other will be 8.5 feet tall.
Aquatica when completed will have more living and storage space that many boats in the 30 to 35 foot range and certainly more headroom than most sailboat and many power boats.
Being tall I needed more space in areas like my shower stall. I've pulled the old 50 gallon gas tank since the boat is powered by electromagnetic water jets it runs on zero fuel. No props or complex engines or parts to maintain. It's clean, quiet, and has no ability to cut up dolphins, manatees, or seagrass beds. It's also legal in all pole and troll only areas.
The gas tank is being replaced by a freshwater tank. The boat harvests rainwater and filters it automatically. The electromagnetic generator will feed a battery bank of 12 deep cycle golf cart batteries using a 10,000 watt inverter for the primary power system. A secondary smaller solar and wind powered bank of 4 batteries will operate lights, or small loads as a backup bank during maintenance of the generator.
I can live comfortably on the hook with more electric power than used by two and a half households or roughly the average consumption of about ten people. Because of this amenities such as tankless water heater, small washer/drier, TV, air conditioning, dehumidifier, convection oven, microwave, bilge pumps, etc etc are all easily operated without any major drain on the system or the need to select what I wish to run. Electromagnetic power is endless free power and easy to build and use.
I think that being a liveaboard is contingent very much on what you make of the space you are working with. I know that a lot of crap will have to go once I move onto Aquatica. It will be some time yet before I complete the build. But asking what is the smallest boat one could live on in reasonable comfort is also a choice of what kind of vessel you prefer, what you are willing to have or omit from your lifestyle, where you plan to live (I'm in Florida so conditions are good much of the time) and whether you are complying with the space that some one else gave you when they designed a boat and you purchased it, or whether you've decided to modify or build your own boat in which case you are dictating what you can or cannot have as space.
At 23' hull length and 152 square feet of living space Aquatica is more than adequate for my needs. That said it might be unacceptable for someone wanting 50+ feet of space.
Just my thoughts on the subject.