There really is no "standard size" for picnic tables in campgrounds that we've seen. Some parks have commercially built tables that vary from 6-8 feet long, while at least one park we visit used to have round concrete tables, but now has rectangular 3'x5' wooden tables, built on site. Other parks we've stayed at had a mixture of rectangular and round tables, some with umbrellas through a center hole. We have a selection of cheap plastic table clothes on board that we can use for most sizes we run into.