The largest (though by no means the worst) tornado ever recorded was the Hallam, Nebraska tornado of May 22, 2004. It was 2.5 miles wide but resulted in only 1 death. Smaller, but still very large...
Presumably you mean to ask how long a tornado lasts. The average duration for a tornado is about 10 minutes. However, tornadoes can last anywhere from just a few seconds to over an hour. The longest...