It wasn't a "small plane", it was a twin-engine Mitchell
B-25 Medium Bomber, fuselage length 53', wingspan 67½', overall height 15'9" exluding its twin vertical stabilizer assemblies, empty weight around 10½ tons, normal operational weight around 17-18 tons including bombload, armament, crew (of 5 to 8 depending on mission), and fuel.
It was lost in fog, travelling relatively slowly (the plane was capable of a maximum airspeed of 275mph at 15,000 feet altitude, considerabley less at sea-level), and was not laden with tens of thousands of gallons of fuel (even in long-range ferry configuration, stripped of armament, with additional bomb bay tanks, waist tanks in place of gun emplacements, and external drop tanks, the plane's maximum fuel capacity was barely 1500 gallons).
The plane that crashed into the Empire State Building