We have spent a lot of time in Yellowstone mainly camping at Fishing Bridge over the years. Site lengths are pretty accurate as there will be someone right behind you. Over the years, they have removed some of the trees that created problems backing into sites. Sites are still tight to get into and you need to have good backing up skills with a larger rig. Always fun watching someone that has poor backing skills and a wife hollering at him get into some of the tighter spaces. They will fit as advertised, just a little patience and skill to get it done.
Once there, you won't spend much time in the campground, so don't worry about how close you are to your neighbors. Go out and enjoy the park, come back and eat then hit the sack as you will most likely be tired from all the activities and effects of the 7000+ altitude.
Keith