If it is a campground I have never been to, I lie.
My hybrid is about 18 feet, but far too often I have booked a CG and told them this only to get the worst spot in the park, even though there are other open sites. Nothing much more than about 20 feet would fit on these sites, so you know they were made to squeeze a pop-up or very small TT on. I've had the dumpster as a neighbor on more than one occasion and it's spot was larger than mine.
Now, if they ask, I have a 29 foot trailer as I'm not a fan of paying the same as everyone else and getting less.