I was going to reply to this thread by quoting the Marchi manuscript, but Andreas beat me to it. i.e. the sound post diameter is dictated by the sound hole width.
If I may be excused telling anecdotes; I recently bought a violin my father made for a leading player in 1954 from his daughter. The violin is recorded in his log book, which I have. The label notes that it is a “copy” of the gentleman's Petrus Guarneri. It doesn’t remind me of any Guarneri, although I expect he would have copied some measurements. As I got it, I was very pleased, and was thinking to myself what nice violins the old man made, and took the strings off to clean 70 years worth of spit and sweat off it. It had a bridge on it with somebody's name stamp that I didn't recognise and a sound post that certainly wasn’t my fathers. The violin, presumably because of the “model”, had rather narrow sound holes, and some primate had forced a sound post of standard diameter through the sound hole, and obviously given up, since it was in incredibly tight, apart from not fitting, to the point where it had made a colossal dent on the inside of the fiddle. Then I noticed that this had caused a sound post crack in the belly. I thought, “Oh bugger, I will have to take it to a violin maker”.
The moral of the story is that anyone who wants his violins to exist long term, should make the f holes AT LEAST 6mm wide, or have a son who will repair it with due care and respect 67 years later.