Unable to answer "What diameter is specified on the sign for this fire hydrant?" – 823 (Pillar Fire Hydrant) – https://osm.org/node/11346101976 via StreetComplete 62.1:

Ist auch kein Schild.

Attached photo(s):
https://streetcomplete.app/p/323844.jpg
https://streetcomplete.app/p/323845.jpg
https://streetcomplete.app/p/323846.jpg