A crucial point is being missed here. The first issue in question is not length.
First, you have to decide what kind of camper you are. If you are the sort who would never consider a site without full hookups (Mon Dieu!), then don't worry a whit about length. Pretty much anyplace you'd accept will have a place to fit you.
If, however, you prefer more rugged, primitive and spread out campsites and can tolerate no hookups, length can be a big limiting factor in many places.
The classic example is Yellowstone. Fishing Bridge is a soulless parking lot and yet there is always big demand to get in. I much prefer Canyon Campground sites (it is nice to have a walkable shower house!), but precious few sites will fit anything over 30'. No hookups.
