I just started a thread on that subject in:
http://www.trawlerforum.com/forums/s3/my-new-tiny-trawler-42353.html
The thread notes that 22-23' is about the minimum to have enough cruising "stuff" on board for overnights.
Duffy makes an electrical powered open cockpit "cocktail cruiser" of about 20'. It uses lead acid batteries and has a range of about 10 miles. You can rent one for an afternoon in places like Newport Beach, Ca.
It would be fairly easy for a builder of 20-25' inboard cruisers like Sisu, my Pompano, Eastern, etc to add a modern electric motor/battery system which would give you a range of maybe 100 miles using Li batteries. A boat like that would have similar capabilities as my Pompano, but it would only go slow, maybe 6 kts max to stay within efficient "hull speed".
But don't expect that solar panels will keep it running. There just isn't enough roof top space, even on boats with long cabins like the Rosborough 246, to put enough panels on to recharge the batteries.
David