Unable to answer "What diameter is specified on the sign for this fire hydrant?" – Fire Hydrant – https://osm.org/node/7770411997 via StreetComplete 57.2:

Nie ma tabliczki, ale sa takie oznaczenia

Attached photo(s):
https://westnordost.de/p/208828.jpg
https://westnordost.de/p/208829.jpg