As you've probably already figured out, everyone's space requirements are different. Most full-timers decide they need more space, but some decide they'd rather have the flexibility of less.
I can't speak for anyone else, but I've been full-timing for 3.5 years (solo) in a 17' travel trailer, and I would never want something larger for the kind of camping I do. Like you, I like state and national park campgrounds that tend to have smaller sites. I prefer a natural atmosphere to the amenities of an RV park, and I chose my RV accordingly. I do a lot of boondocking and like tiny, out of the way places way out in the desert or high in the mountains that big rigs can't get to.
In the end, no one knows what's best for you but you, but all of these varied answers should give you food for thought to come up with the right answer. Best of luck!