The smaller you are, the more places you can park, particularly public campgrounds. What length is critical will depend on where you are. Width when parked is also important too, because not every place has been designed to accommodate multiple slideout configurations popular today.
Most of my RVing has been in the midwest farm belt and on the Great Plains, a mix of commercial RV parks, COE facilties, and state parks. In the commercial parks, and the "improved" campground in public parks, "big" here does not become an issue until over 32-36 feet, e.g. that size you will fit into almost any site, though you might have to park a tow vehicle elsewhere.
But there are other campgrounds in the same parks where a lot of the sites are sized for tent campers, and if you want to fit into one with a RV, the site selection is limited and something around 16-24 feet might be the limit for those.
I come from Michigan, still visit several times a year, not often RVing, but I do go to the state parks. Some of the state park campgrounds are suited to modern RVs, at least to 36 feet if you don't have too many slideouts to fit between the trees. Others are sized from the the 50s and 60s, when almost everyone was tenting or in Pups and RVs not only rare but generally small, under 22 foot. Larger RVs pack into these campgrounds, but they are packed wall to wall. A space that will handle a 36-45 footer with 3-4 slideouts will be scarce, most likely immediately reserved by someone who knows about it.
I can't tell you about the west coast or eastern part of the country. I've been across southern Colorado and to the red rock country of the Colorado Plateau, and my experience there was a lot like on the plains, up to 30-32 feet generally not a problem at commercial parks, maybe more than half the space in public campgrounds also big enough. Much larger than 36 feet you will be looking for a few specials sites (often roadside pullthroughs) made to handle bigger rigs.