According to Official Guinness Records,
The largest banner flown by a vehicle is 383.18 m² (4,123.55 ft²) and was achieved by Ford Motor Company (USA) at the Ford Championship Weekend in Homestead, Florida, USA on 19 November 2016.Ford used a giant American flag as the banner, with a dozen volunteers holding the flag while the driver drove away, letting it unfurl without touching the ground. The record was achieved on a closed course before opening to the public at Ford Championship Weekend at the Miami-Homestead Raceway in Homestead, Florida, USA.
For a complete list of 2016 records, please visit 2016 Guinness Records in United States.