mongodb, get jumbo chunks size

gistfile1.txt

var ns = "mydb.mycollection" //the full namespace of the collection
var key = { "su" : 1, "sd" : 1 } //the shard key of the collection

db.getSiblingDB("config").chunks.find({ns : ns}).forEach(function(chunk) {
        var ds = db.getSiblingDB(ns.split(".")[0]).runCommand({datasize:chunk.ns,keyPattern:key,min:chunk.min,max:chunk.max});
        print("Chunk: "+chunk._id +" has a size of "+ds.size+", and includes "+ds.numObjects+" objects (took "+ds.millis+"ms)")
    }
)