Longest I know is the classic…
Donaudampfschiffahrtsgesellschaftskapitän
…@ 42 letters. Any longer in the video?
Well I suppose I should include the behemoth…
Donaudampfschiffahrtselektrizitätenhauptbetriebswerkbauunterbeamtengesellschaft
…@ 79 letters, but that is defunct now since the organization it names in pre-WW2 Vienna no longer exists. Not under that name anyway—my guess is the sign writers union went on strike over it!
You can't really include German numbers in this discussion, since their length is theoretically infinite as there are no spaces between the individual words—eg English 'twenty two' is zweiundzwanzig, the words being either side of the 'und' which means 'and'. Example:
9,192,631,777
Neunmilliardeneinhundertzweiundneunzigmillionensechshunderteinunddreißigtausendsiebenhundertsiebenundsiebzig—you can count that!
In English, the 28-letter 'Antidisestablishmentarianism' I see in the video is the longest undisputed word I know. But there is a debate whether or not the 45-letter medical term…
Pneumonoultramicroscopicsilicovolcanoconiosis
…is a legit word.