Personally I now think a 20ft is too short, would much rather have a 24ft. If trailers weren't so damn expensive right now I would come off my trailer.
I've had 18 & 20, and used 14-16. I agree with most above that 20 is a decent size. My next one will like be ~22. I would go 24 too, but at some point I'd just pull my 30ft.