My state park experience is Texas, Oklahoma, Missouri, Illinois, Iowa and Michigan. In no place have I encountered an overall rig length limit for the park. Campsite sizes, however, vary a lot, and not all state park campsites are for RVs.
Some of the older state parks in Michigan might have RVs in sites originally developed for tent camping, and large RVs get crowded quite close together, but most have specific campgrounds for modern RVs. Some Oklahoma parks built by the CCC present a similar situation, and the roads into the small-site campgrounds might be difficult for longer RVs to traverse.
In both Texas and Oklahoma, older campgrounds with mostly small back-in sites will often have a few "big rig" RV sites in pullouts parallel to the road through the campground, which tend to be less private.
