Thanks to stackoverflow;
.lbl {display:block}
...
<asp:Label ID="Label1" ClientIDMode="Static" runat="server"
Text="Label" CssClass="lbl"></asp:Label>
...
$(function() {
$( "#label" ).resizable({
resize: function( event, ui ) {
var height = ui.element.height();
ui.element.css({'font-size':height+'%'});
}
});
});