$(this).size() problem

    $("#name_of_div").mousemove(function(){
        var dim = $(this).size();
        var coord = $(this).position();
        $("#test").text( "width: " + dim.width + ", height: " +
dim.height );
    });
I use this cod ... and in div test appear: width: undefine heigh:
undefine .... why ??? Pliz help me
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