The flat type with a finger depression might be better, but they are
hard to find. Otherwise, I think ~15mm is better. What size is that
one? Is it collet type?
This one is 19 mm at the base, tapered a bit, brass insert with
setscrew. It's a little rubbery feeling, nice to grab and spin while
you're looking at something else, like a scope screen. We tested some
of the things with the finger depression and found them fairly awkward
to use... to spin them very far, you wind up swiping them along the
side of the knob a bunch of times, rather then trying to spin the
slippery depression, which challenges my manual dexterity. Not to
mention expensive. Well, OK, I mentioned it.
15 mm would probably work even better, but it's starting to look less
manly.
John