How large might a paper wasp nest become?
According to multiple reports (here, here and here), a 7m (22ft) paper wasp nest was found in an abandoned home in San Sebastian de La Gomera on the island of Tenerife (~100km off the shore from Morocco)!! The nest was estimated to be home to over 22,000 wasps! The picture is included below:

This nest broke the previous world record, a nest discovered in Waimaukau in New Zealand in April 1963 that was an impressive 3.7metres, or 12ft 2ins long, more than 5ft in diameter and 18ft in circumference!



Other big nests? ...


According to this site a 10 ft x 7ft x 2ft nest buzzing with about 400,000 yellow jackets was found in a camper in SC, U.S.
Here is a video of an apparent giant (6.5ft x 8ft) yellow jacket nest found in Florida, U.S. in 2013. A Picture of the nest is below: