What struck me most this time was the sight of two young boys (who looked like they would have been better employed taking part in railway line sports days or exploring deceptively-deep stagnant water) carrying a wicker basket filled with stones, in preparation for the event. When they emptied the basket's contents onto the ground I saw that most of the stones were about an inch in diameter, in a range of shapes - some smooth, some jagged. On the whole, though, they didn't look heavy enough to inflict fatal damage, though they could certainly cause a nasty cut. Was this realistic?
The only other accounts that sprang to mind were in Shirley Jackson's "The Lottery" and the Book of Acts. Jackson tells us that: "Bobby Martin had already stuffed his pockets full of stones, and the other boys soon followed his example, selecting the smoothest and roundest stones." Stones that are small enough to be stuffed several to the pocket are clearly pretty small, and in this case there's not even jaggedness to cause damage. Poor Mrs Hutchinson must have been a long time dying - a fate raising the ghoulish spectre of The Horribly Slow Murderer with the Extremely Inefficient Weapon.
The Book of Acts doesn't add much, but it does mention that "the witnesses laid down their clothes at a young man's feet, whose name was Saul" (7.58). If they had to take their coats off, perhaps chucking those stones was hot work? Obviously, a stone that's too big can't be thrown at all, but a stone that's too small won't inflict sufficient damage. So, is there an optimal size and shape when it comes to stoning?
I got this far, when I remembered that stoning isn't confined to fiction or history, so I clicked on Google images.
About nine inches seems to be the usual choice.