Currently margin and padding can be set in the RadHtmlChart through the TextStyle property exposed by the Axis Label, Axis Titles, Chart Legend, Chart Title and Series Labels. Add the ability to control the margin and padding for the PlotArea as well.
For the time being the following workaround can be used:
JavaScript:
	<script>
		function pageLoad() {
			var chart = $find("<%=RadHtmlChart1.ClientID%>");
			chart._chartObject.options.plotArea.margin = {
				top: 100,
				right: 100,
				bottom: 50,
				left: 100
			};
			chart.repaint();
		}
	</script>
ASPX:
		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="400" Height="400">
			<PlotArea>
				<Series>
					<telerik:PieSeries>
						<SeriesItems>
							<telerik:PieSeriesItem Name="Item 1" Y="30"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem Name="Item 2" Y="10"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem Name="Item 3" Y="20"></telerik:PieSeriesItem>
						</SeriesItems>
					</telerik:PieSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>