> var maxSize = 16*1024*1024;
> var overheadWithId = Object.bsonsize({_id: ObjectId(), str:''});
> var id = ObjectId();
> var str = new Array(maxSize - overheadWithId + 1/*one byte over*/ + 1/*commas are separators*/).toString();
> var obj = {_id: id, str: str};
> assert.eq(Object.bsonsize(obj), maxSize+1);
> db.tooBig.drop();
false
> db.tooBig.runCommand('insert', {documents: [obj]});
{
        "ok" : 1,
        "n" : 0,
        "writeErrors" : [
                {
                        "index" : 0,
                        "code" : 2,
                        "errmsg" : "object to insert too large16777217"
                }
        ]
}
> db.tooBig.runCommand('update', {updates: [{q:obj, u:{$set:{a:1}}, upsert:true}]});
{
        "ok" : 1,
        "nModified" : 0,
        "n" : 1,
        "upserted" : [
                {
                        "index" : 0,
                        "_id" : ObjectId("5314d1f6f0a5a83253a57d04")
                }
        ]
}
> assert.lte(Object.bsonsize(db.tooBig.findOne()), maxSize);
assert: 16777228 is not less than or eq 16777216 : undefined
Error: 16777228 is not less than or eq 16777216 : undefined
    at Error (<anonymous>)
    at doassert (src/mongo/shell/assert.js:11:14)
    at Function.assert.lte (src/mongo/shell/assert.js:293:5)
    at (shell):1:8
2014-03-03T14:03:18.778-0500 Error: 16777228 is not less than or eq 16777216 : undefined at src/mongo/shell/assert.js:13
> db.tooBig.drop();
true
> db.tooBig.runCommand('update', {updates: [{q:{}, u:obj, upsert:true}]});
{
        "ok" : 1,
        "nModified" : 0,
        "n" : 1,
        "upserted" : [
                {
                        "index" : 0,
                        "_id" : ObjectId("5314d1f6f0a5a83253a57d04")
                }
        ]
}
> assert.lte(Object.bsonsize(db.tooBig.findOne()), maxSize);
assert: 16777217 is not less than or eq 16777216 : undefined
Error: 16777217 is not less than or eq 16777216 : undefined
    at Error (<anonymous>)
    at doassert (src/mongo/shell/assert.js:11:14)
    at Function.assert.lte (src/mongo/shell/assert.js:293:5)
    at (shell):1:8
2014-03-03T14:03:18.882-0500 Error: 16777217 is not less than or eq 16777216 : undefined at src/mongo/shell/assert.js:13
> db.tooBig.drop();
true
> db.tooBig.runCommand('update', {updates: [{q:{}, u:{$set:obj}, upsert:true}]});
{
        "ok" : 1,
        "nModified" : 0,
        "n" : 1,
        "upserted" : [
                {
                        "index" : 0,
                        "_id" : ObjectId("5314d1f6f0a5a83253a57d04")
                }
        ]
}
> assert.lte(Object.bsonsize(db.tooBig.findOne()), maxSize);
assert: 16777217 is not less than or eq 16777216 : undefined
Error: 16777217 is not less than or eq 16777216 : undefined
    at Error (<anonymous>)
    at doassert (src/mongo/shell/assert.js:11:14)
    at Function.assert.lte (src/mongo/shell/assert.js:293:5)
    at (shell):1:8
2014-03-03T14:03:18.980-0500 Error: 16777217 is not less than or eq 16777216 : undefined at src/mongo/shell/assert.js:13