As far as length is concerned, there are two versions of the 777 classic series, the -200 and -300. The -300 is the longer one, at 242 ft 4 in (73.86 m). Other than choosing one of these two models, there is no customer option to change the length of the plane. I'm not sure where the Saudia data comes from.
But the 777-300 is not the longest plane in the world. The A340-600 previously held that title, at 247 ft 4 in (75.36 m). The 747-8 surpassed this at 250 ft 2 in (76.3 m). The 777-9 may enter service in 2021, setting a new record of 251 ft 9 in (76.7 m).