The simple answer to the queery is as
follows:-
Before nationalisation (and in the early years of BR) the minimum width of a platform had to be 6 feet from platform edge to the front of any building or fence.
As an example, using this rule a double platform with a seven foot wide building in the middle would have to be a scale 19 feet wide.
Even GWR halts were a minimum of six to seven feet wide to comply with this rule.
Using this formula only gives minimum widths, as in reality the vast majority of platforms were wider than the minimum.
I have no doubt that whatever maximum width I suggest here, someone will be able to come up with an example of wider!
Therefore for maximum go with the rule "if it looks right, it is right" and you'll not have a problem.