Definitely don't choose based on layouts on a website. GO to a dealer or a forum rally and SEE the units you're interested in. Sit on the throne, lie on the bed. In some models the door to the bathroom door opens into the bed. Miserably narrow for a 2 am potty break after having a few draft beers.
Think of spending three days in chilling rain inside.... got enough room to not go stir crazy?
Some people love 19'. I wouldn't go smaller than a 23'. It's all individual preference. What is right for YOU is what counts.