Much said above is true regarding rig size and NPS site access. Other things to consider are width and height. I personally have reserved sites that the length was alright (35 ft) but the site width did not allow for slides due to trees or other fixtures. Additionally, tree limbs over hanging the site can limit the height (it is illegal to perform your own pruning).
I use the web site recreation.gov and enter my requirements to get the appropriate NPS site listings and these are generally correct and will have notes about various restrictions.