Unable to answer "What diameter is specified on the sign for this fire hydrant?" – Pillar Fire Hydrant – https://osm.org/node/6139726114 via StreetComplete_ee 61.1:

Der Hydrant ist ein ganzes Stück woanders.

Attached photo(s):
https://streetcomplete.app/p/330101.jpg
https://streetcomplete.app/p/330102.jpg