Before building an index, customers often want to know 1) how large it will be and 2) how long it will take to build. This ticket captures a feature request for a command, or special option to createIndexes, to return both an estimate of data size and runtime for an index build.

This does not need to be fast, but should return in a relatively constant, reasonable amount of time (say, under a few minutes). Accuracy is more valuable than speed.