The bigger the boat, the more roof space there is for solar panels! It is an impossible question to answer, without knowing your tolerance level for squalor and wish to benefit from the wonders of civilisation, like regular bathing, clean clothes, hot meals, staying warm, light in the evenings etc and where else you might be able to benefit from them, other than the boat. For example, a lived on boat in a full service marina can be smaller than one on a no facilities mooring, or CCing. Also, with two people, or more on a boat, can you cope with not being able to close, or slam a door between you occasionally? A bigger boat means you can. Cabin fever is a thing during long winters.
