The LOC course sector width does depend on runway length, and you are correct that the nominal width is to have full deflection 105 m (350 ft) either side of the threshold*.
The width is, however, limited to six degrees. On shorter runways, this creates a problem which is solved by instead having full deflection equal to the same displacement at point B, which is 1050 m (3500 ft) from the threshold.
This is specified in ICAO Annex 10 Vol I §3.1.3.7.
The 2.5 degree figure often given is just another "close enough and won't confuse the public" figure.
Cheers,
Fred
*) Or the ILS reference datum, if we are to be strict about the definitions. That's where you'll be when crossing the threshold.