My understanding of German grammar is that words can be of infinite length since they allow unlimited compounding. But they also have a language authority that makes words official, and the current longest is 68 letters.
So it would indeed be an interesting exercise in German.
Duden (the standard German dictionary) is the closest I know. And they list "Aufmerksamkeitsdefizit-Hyperaktivitätsstörung" (the German word for attention deficit hyperactivity disorder) with 44 letters as the longest in the dictionary [1].
*Update:* There are also [2] and [3] but they are both not anymore part of a law (the "-gesetz" suffix in the word) or regulation (the "-verordnung" suffix in the word), respectively.
[1] https://www.duden.de/sprachwissen/sprachratgeber/Die-langste... [2] https://en.wiktionary.org/wiki/Rindfleischetikettierungs%C3%... [3] https://en.wiktionary.org/wiki/Grundst%C3%BCcksverkehrsgeneh...