jQueryUI Dialog Position and Size
$( '<div/>' ).load( $this.attr( 'href' ) ).dialog({
  height: 'auto',
  maxWidth: 600,
  position: 'center',
  resizable: false,
  title: $this.attr( 'title' ).length > 0 ? $this.attr( 'title' ) : false,
  width: 'auto',
  resize: function( e, ui ) {
    $(this).dialog( 'option', 'position', 'center' );
  }
});

$( '<div/>' ).attr('id', 'my-dialog').load( 'hello.html' ).dialog({
  height: 'auto',
  maxWidth: 600,
  minWidth: 500,
  position: 'center',
  resizable: false,
  title: $this.attr( 'title' ).length > 0 ? $this.attr( 'title' ) : false,
});

$(window).resize(function(){
    $('#my-dialog').dialog( 'option', 'position', 'center' );
});