@DaveF, thank you for joining the discussion here! After all, it’s a continuation of your former discussion with @dieterdreist.
It is indeed defined by entry signals (that’s why they are called that way). Just see the Hungarian railway code I linked in the first comment. (It’s not even the German one!) Translation:
1.2.4. Station area
The area between either entry signals placed at the ends of the station bordering the open line or V-letter signals or control signals in the direction of entrance. At termini, the area within the entry signal or V-letter signal or control signal in the direction of entrance.
You also said this on the Wiki:
A railway station is a place accessible to the public. It doesn’t stretch a whole kilometre down the tracks, it doesn’t include sidings (And by that I mean the genuine meaning of sidings. Tracks which run adjacent to platforms allowing passengers to catch a train are /not/ sidings).
The area between entry signals of a station can stretch a whole kilometre—often even more than that. Just check out this Overpass query.
And a railway station does include sidings—see the Train station Wikipedia page or the station manual of any Hungarian railway station.