I've done a pretty extensive study of campground characteristics and I would offer the following reinforcement to several other comments posted. Most private campgrounds are pretty good about being clear if they are "big rig" friendly, but several have not made my day.... so check the web and call ahead if you are concerned. Also, I have found federal and state campgrounds typically have more severe limitations so I always research these. The state and federal websites have proven a good resource for me. (37 foot class A with trailer).
I did a check recently of more than 200 campgrounds around Yosemite and Sequoia national parks and took a different approach in tallying up how many of them might a certain length RV be able to stay there. I say might as a campground may take up to 40' RV's but have fewer than 10 spaces of this size and as they are first come first serve other RV's including shorter ones may take all the spaces.
Many campgrounds do not take reservations and with these it becomes more important to be able to fit your RV into the spaces still available when you arrive.
The shorter you are the more places you can get into.
We have a 36-1/2' FW and we can get into my favorite CA campgrounds, including state and National Parks. We wouldn't go bigger unless we were full-timing. Not sure if you have kids, but don't go too small. Our last FW was a 27' and with 2 kids, it was used more for storage and sleep.
As others have said, it's a "campground thing", not a "state thing". Other than total combined length and weight restrictions for operating the vehicle on the roadways, there's no law that says RVs are restricted to XX feet in a particular state's campgrounds. If a campground has length restrictions, it's either because their facility isn't designed to handle RVs over a certain length or the roads leading to the campgrounds are narrow and/or twisty or have low clearance.
Many state parks have length restrictions, not because of laws, but because of limited space sizes and access roads. Most of them were built in an era when a 25' Airstream was a beast of an RV. People weren't hauling around 40' 5th wheels with slideouts.
As for how big to go, you'll figure it out after you've bought your rig. I bought mine for full-timing and decided 35' would be as small a space as I could stand to live in. Absolute minimum. No question. Having lived in my 36' for a month or so, I now know that I could have fit in 30', no problem and 32' would be perfect with the right layout. I've got cabinets and drawers I've never opened. But I certainly don't regret having the extra space. Better to have too much than not enough.