Unable to answer "What diameter is specified on the sign for this fire hydrant?" – Pillar Fire Hydrant – https://osm.org/node/6372813044 via StreetComplete 62.2:

Der Hydrant besitzt kein H-schild. 
2 m weiter steht das Schieberschild

Attached photo(s):
https://streetcomplete.app/p/326858.jpg