Unable to answer "What diameter is specified on the sign for this fire hydrant?" – Pillar Fire Hydrant – https://osm.org/node/9295610962 via StreetComplete 62.2:

Einziges klassisches Schild steht einige Meter weiter.

Attached photo(s):
https://streetcomplete.app/p/327438.jpg
https://streetcomplete.app/p/327439.jpg