Getting the file size of a file in javascript

<input id="archivo" name="archivo" type="file">

So my field of type file is called archivo which parent is formIncidencia, so what i’m doing to find it is:

$('#formIncidencia').find("[id='archivo']");

This works fine but when i’m trying to do:

 $('#formIncidencia').find("[id='archivo']").files[0].size; 

it’s not working like that, and i’m uploading a file so don’t know what it’s happening..