In our environment, the ability to determine array and document sizes without transferring them over the wire would be very welcome. I guess that existing syntax for queries ($size) would do nicely.
I actually wrote this request after I've read someone other's message in mongodb-user: http://groups.google.com/group/mongodb-user/browse_thread/thread/159eecb36444f402 and understood that I'm not the only one with such needs.