My favorite longest word from middle school was Antidisestablishmentarianism. If that word ever appeared on a postage stamp it would beat Hochgeschwindigkeitsverkehr by one letter. :-)
A world record was recently set for the most words on a single postage stamp. Belgium issued a stamp in June 2024 to celebrate the 150th anniversary of the UPU. The stamp includes 2,024 words in microprint.


The "words" are the names of the UPU member nations, and since there aren't 2,024 of them, the names repeat a few times. Still, quite a feat for the engraver.
https://www.guinnessworldrecords.co...ostage-stamphttps://press.bpost.be/bpost-breaks...al-union-upuEdit: I originally counted the letters of "Hochgeschwindigkeitsverkehr" from NSK's post. With the added 's' at the end of "Hochgeschwindigkeitsverkehrs" from the OP, this word is tied with the number of letters in "Antidisestablishmentarianism".