It depends on what your own requirements are. We were in Utah and saw an older couple, 80"s, who were living in a Van. Make shift bed, cook outside, stayed only in parks next to restrooms and showers. (don't think they used the showers very often). You could just barely get in to drive it.
Another older couple are living in an Airstream trailer about a 1970 model, 18-20' long,,, full time in Thousand trails, 2 weeks here, 2 weeks someplace else and were very happy. Last time I saw them they had been in it for about 8 years.
So size is the amount of space you require to be comfortable.
Jim M