For the time being you can use the following workaround:
CSS:
	<style>
	 .RadNotification .rnContentWrapper .rnIcon:before {
			font-size: 0.7em\9;
		}
		_:-ms-fullscreen, :root .RadNotification .rnContentWrapper .rnIcon:before {
			font-size: 0.7em;
		}
	</style>
ASPX:
		<telerik:RadNotification ID="RadNotification1" runat="server" AutoCloseDelay="99999" ShowInterval="1000" Position="Center" RenderMode="Lightweight" Text="Project deleted successfully!" VisibleTitlebar="false">
		</telerik:RadNotification>