.840" is probably the most common size. I've seen them vary by as much as .040" either way, but most are probably between .83 and .86 with the vast majority between .84 and .85.
The Schuler joint is outside the normal range at 22mm diameter, which is about 0866”. It is or was a common carom cue joint diameter, and it makes a bit more sense in the metric world.
