Best way to determine actual H3 hexagon size?

What is the best way to get the actual size of an H3 hexagon (area or edge length) given an H3Index and not the average constant provided by the H3 API?

Update: H3 now includes functions for exact area and edge length measurements - documentation here.

Original Answer: At present, the H3 library doesn't expose any functions for this (though we've considered it, and may add in the future). At present, we've decided that the H3 library is no better placed to calculate pure geographic areas/distances/etc than any other geo library, so the best option is to find another geo library in your language of choice and apply it to the output of h3ToGeoBoundary.

You can see an example of doing this in JavaScript in this Observable notebook.