How big really is 0.5 mM?

So I find myself asking that question today.  In relative terms it is such a tiny length that it is hardly noticeable.  In fact, I'd be hard pressed to be able to distinguish two every day items if the only difference between them was 0.5 mM in length, however, when it comes to precision, 0.5 mM can be the determinant between amazing success or catastrophic failure.  Could you imagine what would happen if the support beams for the space shuttle were that uneven..and at the same time, physicists are able to accurately collide subatomic particles where 0.5 mM might as well be the atlantic ocean.  Why this sudden curiosity over 0.5 mM today, well, apparently, 0.5 mM is the difference in acrylamide gel thickness that I can successfully work with with out tearing it into pieces.  Better luck next time.