Currently, prior to building an Atlas Search index, there is no way for users to know how large that index can get. Knowing the projected-index-size in advance can especially be helpful for users who might need to build index on a large collection, and where building such an index can push the disk-utilization to the max.

Therefore, this is a feature request to implement a way to "forecast the Atlas Search index-size" in advance, before starting the index build.