Antonov An-225, 275 ft 7 in (only one example was ever completed).

Runners up:

Boeing 747–8, 250 ft 2 in (128 of this version of the 747 were built)
Airbus A350–1000, 242 ft 8 in (8 of this version of the A350 have been built so far)
Boeing 777–300, 242 ft 4 in (853 of this version of the 777 have been built so far)
Airbus A380, 238 ft 7 in (229 of these have been built so far)Boeing 777X, 251 ft 9 in

Hope you got the answer you are looking for thank-you.