I don't know about the campsite sizes or availability, but I assume you already know that there are no hookups and only cold showers in all the Cape Hatteras National Seashore NPS campgrounds. Of course, that's one of the reasons why they're less expensive.
The NPS web site also indicates that only Cape Point, Ocracoke and Oregon Inlet campgrounds have dump stations nearby.
2009 GMC Sierra 2500HD | Crew Cab | Standard Box | 4WD | Duramax/Allison
2000 Jayco Eagle 266 | FBS | TT
1986 Coleman Laramie pop-up -- Still in the family!!!