The only NPS campground that we've run into that actually measures is Organ Pipe (they have lines painted on the pavement at the entrance). We've run into several with posted size limits. Some, we could easily tell why there were limits (small sites, tight bends). Others with limits, we had no idea why they had the limits. We were once told that they listed the smallest site as the size limit so that if that was the only one left, no one could complain that it was too small.
Reserveamerica.com lists site sizes for many federal and state campgrounds and even has site pictures for some. It can come in handy.
We have lied on size when making a reservation but as Gary said, if you lie and you're too big for the site, don't expect sympathy, assistance, or a refund.