Unable to answer "What diameter is specified on the sign for this fire hydrant?" for https://osm.org/node/5851739318 via StreetComplete 45.0:

Vier Erdgas-Schilder

Attached photo(s):
https://westnordost.de/p/102694.jpg
https://westnordost.de/p/102695.jpg
https://westnordost.de/p/102696.jpg