Based upon a 12 m long bus, they say the normal length is 23 m, the absolute minimum 19 m (if there is no adjoining parking), but that 37 m should be allowed for manoeuvring in and out if there is adjoining parking.
TSM Chapter 3, para 17.22 says:
“Where the stop serves frequent or multiple services, or vehicles with different entrance positions, the length of the bay may be increased in increments of 2 m. The legend BUS STOP should be used once for every complete length of 12 m. Under-used stops of excessive length may result in enforcement difficulties.”
So that would infer that the minimum length would be somewhere between 12m and 22m, as you don’t often see “BUS STOP” written twice within the marking.
The TfL guidance is very good. 19m is the absolute minimum assuming there is clear entrance and exit to the stop (i.e. a clearway or double reds/yellow).
A bus will not be able to pull adjacent and close to the kerb if there are obstructions before or after, meaning mobility impaired passengers will struggle to board/alight and you will have failed your duties under the Equality Act.