Unable to answer "What diameter is specified on the sign for this fire hydrant?" – Pillar Fire Hydrant – https://osm.org/node/10100802986 via StreetComplete 62.1:

Da ist kein Schilf. Es steht drauf dn100

Attached photo(s):
https://streetcomplete.app/p/322503.jpg
https://streetcomplete.app/p/322504.jpg