This is an advertisement, but it's one of the few I actually enjoy watching, and it suggests a track is "2500 times smaller than a human hair" which puts an upper bound on the size of a bit: https://www.youtube.com/watch?v=ZXs_9OXRnQo
This doesn't answer your question but your question made me think of this and I thought I'd share for anyone else.
This doesn't answer your question but your question made me think of this and I thought I'd share for anyone else.