Unable to answer "What diameter is specified on the sign for this fire hydrant?" for https://osm.org/node/3122224406 via StreetComplete 55.0:

DIN 100

Attached photo(s):
https://westnordost.de/p/186654.jpg
https://westnordost.de/p/186655.jpg
https://westnordost.de/p/186656.jpg