Over 4 football fields across is sizeable but not an extinction event. Depending on the actual mass and impact speed, it would probably be between the equivilent of about a 40 and a 100 megaton explosion. The biggest nuke ever set off was 100 megatons, by the USSR. Those crazy Russians wiped out a good portion of Siberia just to see how big they could make one.
Tzar Bomba was 50 MT. It was scalable to 100 but they never tested it.