Every national park, state park, and federal park has different site sizes. When you make reservations in federal parks you can view a list of all the available sites. The site size is displayed so you can figure out if are any where you will fit. Many state parks also show the size. We camp in a 35 foot fifth wheel and have camped in many state and Corps of Engineers parks without any problems. When I am in doubt about a site, I call the park directly. We have camped in a few national parks including Zion and Everglades.
Good luck! Happy trails!