The _size field allows to automatically index the size of the original _source indexed. By default, it’s disabled. In order to enable it, set the mapping to:

{
    "tweet" : {
        "_size" : {"enabled" : true}
    }
}

In order to also store it, use:

{
    "tweet" : {
        "_size" : {"enabled" : true, "store" : true }
    }
}