How do I say supercalifragilisticexpialidocious in Russian?

Суперархиэкстраультрамегаграндиозный :)

Though this word is not used in everyday life. Basically, there is no one-word translation of this word and in the film it’s translated as “слово, которое говорят, когда не знают, что сказать” (“the word you say when you don’t know what to say”).