Please As an alternative (or in addition), the actual size of the array should be returned.

Motivation:
Related to SERVER-18207:
Our application parses data from MongoDB repositories that are populated by other application. Documents can have potentially large ARRAYS objects at arbitrary points. The application retrieve the data, present the user with "summary" (e.g., show the first 3 values of each array followed by "..."). The full array is displayed once the user choose a specific document, and part of the document.

The above change will allow the application to locate all the arrays in a specific document, and form queries for the required amount of data (using array slices) from each array.