What does the idiom 3 miles long and smaller than human hair mean?

The idiom "3 miles long and smaller than human hair" is not a real idiom. It's a nonsensical phrase that is likely used for humorous effect, often to highlight a contradiction or absurdity.

It plays on the idea of something being incredibly long and incredibly small at the same time, which is physically impossible. It might be used in situations like:

* To describe something exaggerated: "The line at the store was 3 miles long and smaller than human hair!" (meaning the line was very long, but the speaker is exaggerating).

* To poke fun at someone's description: "They said the cake was 3 miles long and smaller than human hair. I'm not sure what they were thinking." (meaning the description is nonsensical).

If you're trying to remember a specific idiom, it's possible you've misremembered the phrasing. Could you provide more context or a similar phrase that might help clarify the meaning?