There is no right answer. There is no right size.
In the end, you have to consider the pros and cons of each size. What works for one poster would be unacceptable for another. How do you want to use it? Do you plan on spending all day cruising around the city and then heading back to the park for the night? Do you plan on stopping while on the road and then settling in at the park? Do you plan on setting up the sewer when you camp or just do a dump at the end of the stay? Your particulars, and what you will be comfortable with, will identify the right length.
For me, I love the 21 ft length. It works well for the way I use my RV. I sleep in the cabover bunk with no problem. If I want to go into town, I just drive off (I dry camp mostly).