 <p>Working for MongoID 5.x. With the mongo shell and MongoID is straightforward:</p> <ol> <li><p>Pick up your object id</p> <p><code>ModelName.first.id # then copy your_id</code></p></li> <li><p>Run mongo console</p> <p><code>mongo</code></p></li> <li><p>Get the size in bytes of the document</p> <p><code>Object.bsonsize(db.modelname.find({"_id": ObjectId("your_id")}))</code></p></li> </ol>