One friend of mine, when she began dating her now-husband, had Rule One laid down
to her immediately: never, never go like this [holds hands about five
inches apart] at a bar. No matter what it is you're talking about: fish,
waiting times, the length of a movie, no matter. If you must gesture, always hold
hands far, far apart.
I presume indicating sizes with thumb and
forefinger is even more strongly verboten.