I'd say it's determined by how bad the speaker's breath is...
What determines speaker distance?
What determines the spacing between the speakers? I've seen a 1.3 formula for setting the distance between each. If I have a 9 foot distance from the speakers to the listening chair that would make the spacing approx 7 feet? Seems a little wide to get a good soundstage. Are there are any other "general" guidelines for getting a basic spacing between the two?