my experience is that the method to specify max length varies all over the place. some places it is pad length some places it's by someone (with or without trailer experience) saying what THEY think will fit. And lots in between.
I've found many places that say 30 ft max trailer and I can park the trailer hooked to my truck (65 ft overall) and fit on the pad. rear wheels hit the back of the pad and the rest of the trailer hangs over.
Other places say 45ft or more max and I have to unhook and put the truck sideways. One favorite campground site has a warning that trailers over 20ft will NOT fit in site x. My 35 ft trailer fits will room to spare and with easy access. Pad length alone is almost 70 ft. Some other sites at the same campground say 30, 35, 40 ft and it is a very tight fit to get a trailer that size into the site given how narrow the access road is in that spot and how tight the pad access is.
I take the info with a grain of salt. many sites let you see the pad and access on the web and along with a phone call I can determine if I'll fit.