Get the size of list of documents in mongo db
As per MongoDB jira blog documentation Real document size in collection and also MongoDB documentation here the Object.bsonsize(&lt;document&gt;) prints the BSON size of a &lt;document&gt; in bytes.

To find the size of all documents in a collection:

Object.bsonsize(db.collection.find( {dbname:"collectionname"}))


For Example 

&gt; use test
switched to db test
&gt; db.createCollection("StackExchange");
{ "ok" : 1 }

&gt; show collections
StackExchange
&gt;

&gt; db.StackExchange.insertOne({"Name" : "user3138864", "Password" : "abc123"})
{
        "acknowledged" : true,
        "insertedId" : ObjectId("5b740353194beb82d3752206")
}
&gt; db.StackExchange.findOne()
{
        "_id" : ObjectId("5b740353194beb82d3752206"),
        "Name" : "user3138864",
        "Password" : "abc123"
}
&gt; db.StackExchange.find().pretty()
{
        "_id" : ObjectId("5b740353194beb82d3752206"),
        "Name" : "user3138864",
        "Password" : "abc123"
}
&gt; Object.bsonsize(db.StackExchange.find( {m040:"StackExchange"}))
45734


For further your ref here and here