From reading these forums for many, many years now and having this same similar question asked many times, I have come to a conclusion.
If you are primarily camping anywhere West of the Rockies, a 25 foot travel trailer will accommodate just about anywhere.
If you are primarily camping anywhere East of Kansas, a 35-40 foot travel trailer can be accommodated almost anywhere, provided you check each individual campground first. Most will accomodate trailers of that length and sites available. But not necessarily in thick wooded, jungles.
National Forests everywhere are primarily designed for around 25-30 foot trailers, and there are some exceptions for longer sites, so check ahead first.