The _size field allows to automatically index the size of the original _source indexed (not the compressed size, if compressed). By default, its 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" : "yes"}
}
}