what is the max size RV in parks?

Lori, I think that really depends on the park where you want to visit. Many of the national park campgrounds are accessible to RVs, though note that there are very few with pull-through sites (as in, you'll have to back in). In Rocky Mountain National Park, RV length limit is 40 feet; in Acadia's Blackwoods and Seawall campgrounds, it's 35 feet (there are height restrictions, too). Best advice, then, is to check limits with parks, forests, and other public lands where you hope to travel in your RV. Happy travels!