It seems like it's usually Rhode Island or Manhattan in the metaphor, so thankfully both absent here. Also, turns out that lots of things are "smaller than a human hair" -- though not in this case. The Grand Canyon itself is also frequently used as a reference, and makes an appearance in this story.
A bullet, though, is a pretty good reference. Small, harmless in its resting state, but violent when up to speed. You know you're fucked if you get hit by a bullet, or anything moving at bullet velocity. If you get hit by something much larger than a bullet moving at bullet speed (debris from a cosmic impact, say), doubly so.