Nine millimeters is .3543 of an inch, approximately the width of the cap of a cheap ball-point pen. As a measure, it's not large nor interesting—unless it is the bore of the barrel of a hit man's firearm. Nine millimeters then becomes huge and fascinating. It holds your attention, fixing you like an insect pinned to a board and leaving you, like that insect, with as little hope of escape.
I must ask Mr. Reese how he keeps his wits in such circumstances. I expect the answer will have something to do with familiarity and experience.