I've been looking at the same thing, and we like mainly state parks, so I went to the reservation site for numerous parks in GA and FL that we like to go to, and put in the camper length to see how many sites I'd fit in. I noticed that 36 eliminated a good many, but then 40+ cut it way down. There we not sites for over 40 in 2 of our favorite parks.
Play around on the different reservation sites, and see how many sites work for the different lengths.
I've decided not to go over 36, and wouldn't mind staying a little below that.