The long and short of it

floccinaucinihilipilification, at 29 letters, as recorded in The Guinness Book of Records in 1992 and subsequent editions.

supercalifragilisticexpialidocious, the song title from the movie Mary Poppins, has 34 letters and appears in several dictionaries as a proper noun. It is defined only in reference to the song title and is hence dismissed as a ‘real’ word. The longest word ever to appear in a non-technical dictionary of English is PNEUMONOULTRAMICROSCOPICSILICOVOLCANOCONIOSIS
(Oxford English Dictionary, 1936).