From looking t the website and photos, I would say when the jaws are fully closed, the "true" diameter of the collet pads is 3/4" +/- a VERY tight machining tolerance at fully closed. I would guess something like +/-.002 or so, maybe less. This diameter would only apply to the straight section of the pads however, not the serrations that are designed to bite into the workpiece to hold the work. Those appear to be diametrically much smaller (assumed in the waste area of your workpiece). this is much like fixed diameter tooling for metal lathes. As for the max diameter, thats completely dependent on your chuck opening, however as you increase from 3/4" diameter, the gripping surface is no longer a true circle.