It is a rare park that will limit you to 20', however, many state parks were built by the CCC back in the 30s and you may well have trouble with something over 30'. I have travelled in about 40 states with longer trailers and have had very few problems.
Until I bought my 22' Airstream about 45 days ago, I towed a TrailManor which was 28' when raised. I never had a bit of touble, although I had to look for longer sites and hang the end of the trailer way out past the pad in some places. This is usually only a problem when the park is pretty full; otherwise, there is most always at least one suitable space available.
I can think of only two places I've visited that something over 20' would be a major problem. One was at Fort Pillow in Tennessee where I had to weave around tight curves, between pines, to get to a site (I had a 19' Scamp 5th-wheel then and even that was tight). The other place is the basin at Big Bend where there is signage advising against taking trailers over 20'. Having been there several times, I have observed many bigger trailers up there and I personally wouldn't hesitate to take my 22' International up there.
To sum it up, I would expect very few problems in state parks with a something in the 25-foot range. Up around 30', you may very well run into a lot of limits on older state and national parks and be pretty well limited to newer parks or commercial parks.
Just my opinion.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10